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volodimyrvolodimyr
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postorder/preorder traversal BST
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‎144.binary_tree_preorder_traversal/preorder_traversal.go‎

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// 144. Binary tree preorder traversal
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// Topics: 'Stack', 'Tree', 'Binary Tree', 'Depth-First Search'
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// Given the root of a binary tree, return the preorder traversal of its nodes' values.
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// Example 1:
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// Input: root = [1,null,2,3]
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// Output: [1,2,3]
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// Explanation:
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// Example 2:
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// Input: root = [1,2,3,4,5,null,8,null,null,6,7,9]
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// Output: [1,2,4,5,6,7,3,8,9]
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// Explanation:
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// Example 3:
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// Input: root = []
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// Output: []
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// Example 4:
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// Input: root = [1]
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// Output: [1]
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// Constraints:
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// The number of nodes in the tree is in the range [0, 100].
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// -100 <= Node.val <= 100
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package binarytreepreordertraversal
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func preorderTraversal(root *TreeNode) []int {
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arr := []int{}
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traversal(root, &arr)
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return arr
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}
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func traversal(root *TreeNode, arr *[]int) {
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if root == nil {
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return
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}
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*arr = append(*arr, root.Val)
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traversal(root.Left, arr)
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traversal(root.Right, arr)
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}
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type TreeNode struct {
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Val int
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Left *TreeNode
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Right *TreeNode
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}

‎144.binary_tree_preorder_traversal/preorder_traversal_test.go‎

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package binarytreepreordertraversal
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import (
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"reflect"
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"testing"
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)
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func TestPreorderTraversal(t *testing.T) {
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tests := []struct {
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name string
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root *TreeNode
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want []int
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}{
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{
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name: "Example 1: Right-skewed with left child",
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root: &TreeNode{
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Val: 1,
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Right: &TreeNode{
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Val: 2,
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Left: &TreeNode{Val: 3},
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},
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},
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want: []int{1, 2, 3},
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},
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{
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name: "Example 2: Complex tree",
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root: &TreeNode{
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Val: 1,
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Left: &TreeNode{
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Val: 2,
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Left: &TreeNode{
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Val: 4,
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},
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Right: &TreeNode{
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Val: 5,
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Left: &TreeNode{Val: 6},
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Right: &TreeNode{Val: 7},
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},
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},
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Right: &TreeNode{
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Val: 3,
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Right: &TreeNode{
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Val: 8,
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Left: &TreeNode{Val: 9},
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},
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},
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},
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want: []int{1, 2, 4, 5, 6, 7, 3, 8, 9},
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},
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{
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name: "Example 3: Empty tree",
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root: nil,
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want: []int{},
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},
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{
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name: "Example 4: Single node",
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root: &TreeNode{Val: 1},
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want: []int{1},
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},
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{
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name: "Two nodes - left child only",
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root: &TreeNode{
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Val: 1,
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Left: &TreeNode{Val: 2},
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},
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want: []int{1, 2},
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},
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{
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name: "Two nodes - right child only",
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root: &TreeNode{
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Val: 1,
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Right: &TreeNode{Val: 2},
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},
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want: []int{1, 2},
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},
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{
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name: "Three nodes - complete binary tree",
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root: &TreeNode{
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Val: 1,
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Left: &TreeNode{Val: 2},
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Right: &TreeNode{Val: 3},
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},
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want: []int{1, 2, 3},
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},
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{
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name: "Left-skewed tree",
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root: &TreeNode{
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Val: 4,
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Left: &TreeNode{
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Val: 3,
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Left: &TreeNode{
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Val: 2,
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Left: &TreeNode{Val: 1},
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},
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},
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},
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want: []int{4, 3, 2, 1},
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},
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{
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name: "Right-skewed tree",
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root: &TreeNode{
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Val: 1,
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Right: &TreeNode{
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Val: 2,
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Right: &TreeNode{
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Val: 3,
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Right: &TreeNode{Val: 4},
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},
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},
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},
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want: []int{1, 2, 3, 4},
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},
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{
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name: "Balanced binary tree",
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root: &TreeNode{
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Val: 1,
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Left: &TreeNode{
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Val: 2,
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Left: &TreeNode{Val: 4},
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Right: &TreeNode{Val: 5},
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},
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Right: &TreeNode{
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Val: 3,
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Left: &TreeNode{Val: 6},
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Right: &TreeNode{Val: 7},
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},
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},
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want: []int{1, 2, 4, 5, 3, 6, 7},
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},
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{
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name: "Tree with negative values",
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root: &TreeNode{
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Val: 0,
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Left: &TreeNode{Val: -50},
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Right: &TreeNode{Val: 50},
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},
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want: []int{0, -50, 50},
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},
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{
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name: "Tree with boundary values",
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root: &TreeNode{
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Val: 0,
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Left: &TreeNode{
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Val: -100,
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Left: &TreeNode{Val: -99},
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Right: &TreeNode{Val: -98},
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},
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Right: &TreeNode{
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Val: 100,
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Left: &TreeNode{Val: 99},
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Right: &TreeNode{Val: 98},
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},
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},
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want: []int{0, -100, -99, -98, 100, 99, 98},
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},
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{
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name: "Unbalanced tree - heavy left",
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root: &TreeNode{
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Val: 5,
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Left: &TreeNode{
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Val: 3,
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Left: &TreeNode{
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Val: 1,
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Left: &TreeNode{Val: 0},
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Right: &TreeNode{Val: 2},
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},
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Right: &TreeNode{Val: 4},
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},
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Right: &TreeNode{Val: 6},
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},
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want: []int{5, 3, 1, 0, 2, 4, 6},
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},
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{
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name: "Unbalanced tree - heavy right",
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root: &TreeNode{
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Val: 1,
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Left: &TreeNode{Val: 0},
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Right: &TreeNode{
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Val: 3,
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Left: &TreeNode{Val: 2},
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Right: &TreeNode{
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Val: 5,
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Left: &TreeNode{Val: 4},
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Right: &TreeNode{Val: 6},
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},
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},
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},
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want: []int{1, 0, 3, 2, 5, 4, 6},
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},
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{
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name: "Deep left subtree only",
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root: &TreeNode{
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Val: 10,
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Left: &TreeNode{
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Val: 5,
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Left: &TreeNode{
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Val: 3,
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Left: &TreeNode{
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Val: 1,
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Left: &TreeNode{Val: 0},
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},
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},
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},
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},
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want: []int{10, 5, 3, 1, 0},
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},
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{
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name: "Zigzag pattern",
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root: &TreeNode{
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Val: 1,
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Left: &TreeNode{
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Val: 2,
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Right: &TreeNode{
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Val: 3,
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Left: &TreeNode{
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Val: 4,
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Right: &TreeNode{Val: 5},
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},
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},
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},
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},
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want: []int{1, 2, 3, 4, 5},
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},
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{
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name: "Maximum nodes (100 nodes would be too large, using smaller)",
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root: &TreeNode{
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Val: 1,
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Left: &TreeNode{
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Val: 2,
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Left: &TreeNode{Val: 3},
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Right: &TreeNode{Val: 4},
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},
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Right: &TreeNode{
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Val: 5,
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Left: &TreeNode{Val: 6},
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Right: &TreeNode{Val: 7},
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},
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},
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want: []int{1, 2, 3, 4, 5, 6, 7},
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},
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{
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name: "All same values",
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root: &TreeNode{
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Val: 1,
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Left: &TreeNode{
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Val: 1,
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Left: &TreeNode{Val: 1},
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Right: &TreeNode{Val: 1},
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},
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Right: &TreeNode{
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Val: 1,
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Left: &TreeNode{Val: 1},
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Right: &TreeNode{Val: 1},
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},
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},
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want: []int{1, 1, 1, 1, 1, 1, 1},
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},
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}
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for _, tt := range tests {
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t.Run(tt.name, func(t *testing.T) {
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got := preorderTraversal(tt.root)
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if !reflect.DeepEqual(got, tt.want) {
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t.Errorf("preorderTraversal() = %v, want %v", got, tt.want)
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}
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})
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}
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}

‎145.binary_tree_postorder_traversal/postorder_traversal.go‎

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// 145. Binary Tree Postorder Traversal
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// Topics: 'Stack', 'Tree', 'Binary Tree', 'Depth-First Search'
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// Given the root of a binary tree, return the postorder traversal of its nodes' values.
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// Example 1:
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// Input: root = [1,null,2,3]
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// Output: [3,2,1]
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// Explanation:
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// Example 2:
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// Input: root = [1,2,3,4,5,null,8,null,null,6,7,9]
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// Output: [4,6,7,5,2,9,8,3,1]
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// Explanation:
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// Example 3:
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// Input: root = []
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// Output: []
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// Example 4:
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// Input: root = [1]
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// Output: [1]
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// Constraints:
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// The number of the nodes in the tree is in the range [0, 100].
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// -100 <= Node.val <= 100
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package binarytreepreordertraversal
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func postorderTraversal(root *TreeNode) []int {
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arr := []int{}
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traversal(root, &arr)
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return arr
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}
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func traversal(root *TreeNode, arr *[]int) {
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if root == nil {
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return
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}
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traversal(root.Left, arr)
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traversal(root.Right, arr)
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*arr = append(*arr, root.Val)
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}
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type TreeNode struct {
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Val int
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Left *TreeNode
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Right *TreeNode
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}

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