Kth smallest element in a BST

Author: volodimyr

230.kth_smallest_element_in_a_BST/kth_smallest.go

@@ -0,0 +1,48 @@
+// 230. Kth smallest element in a BST
+// Topics: 'Tree', 'Binary Search Tree', 'Binary Tree', 'Depth-First Search'
+// Level: 'Medium'
+
+// Given the root of a binary search tree, and an integer k, return the kth smallest value (1-indexed) of all the values of the nodes in the tree.
+
+// Example 1:
+
+// Input: root = [3,1,4,null,2], k = 1
+// Output: 1
+
+// Example 2:
+
+// Input: root = [5,3,6,2,4,null,null,1], k = 3
+// Output: 3
+
+// Constraints:
+
+//     The number of nodes in the tree is n.
+//     1 <= k <= n <= 104
+//     0 <= Node.val <= 104
+
+package kthsmallestelementinabst
+
+func kthSmallest(root *TreeNode, k int) int {
+	sm := -1
+	find(root, &k, &sm)
+	return sm
+}
+
+func find(root *TreeNode, k *int, sm *int) {
+	if root == nil || *k == 0 {
+		return
+	}
+	find(root.Left, k, sm)
+	if *k > 0 {
+		*sm = root.Val
+		*k--
+	}
+	find(root.Right, k, sm)
+	return
+}
+
+type TreeNode struct {
+	Val   int
+	Right *TreeNode
+	Left  *TreeNode
+}

230.kth_smallest_element_in_a_BST/kth_smallest_test.go

@@ -0,0 +1,192 @@
+package kthsmallestelementinabst
+
+import "testing"
+
+func TestKthSmallest(t *testing.T) {
+	tests := []struct {
+		name string
+		root *TreeNode
+		k    int
+		want int
+	}{
+		{
+			name: "Example 1: k=1, smallest element",
+			root: &TreeNode{
+				Val: 3,
+				Left: &TreeNode{
+					Val:   1,
+					Right: &TreeNode{Val: 2},
+				},
+				Right: &TreeNode{Val: 4},
+			},
+			k:    1,
+			want: 1,
+		},
+		{
+			name: "Example 2: k=3, middle element",
+			root: &TreeNode{
+				Val: 5,
+				Left: &TreeNode{
+					Val: 3,
+					Left: &TreeNode{
+						Val:  2,
+						Left: &TreeNode{Val: 1},
+					},
+					Right: &TreeNode{Val: 4},
+				},
+				Right: &TreeNode{Val: 6},
+			},
+			k:    3,
+			want: 3,
+		},
+		{
+			name: "Single node tree",
+			root: &TreeNode{Val: 1},
+			k:    1,
+			want: 1,
+		},
+		{
+			name: "Two nodes, k=1",
+			root: &TreeNode{
+				Val:  2,
+				Left: &TreeNode{Val: 1},
+			},
+			k:    1,
+			want: 1,
+		},
+		{
+			name: "Two nodes, k=2",
+			root: &TreeNode{
+				Val:  2,
+				Left: &TreeNode{Val: 1},
+			},
+			k:    2,
+			want: 2,
+		},
+		{
+			name: "Right-skewed tree, k=1",
+			root: &TreeNode{
+				Val: 1,
+				Right: &TreeNode{
+					Val: 2,
+					Right: &TreeNode{
+						Val:   3,
+						Right: &TreeNode{Val: 4},
+					},
+				},
+			},
+			k:    1,
+			want: 1,
+		},
+		{
+			name: "Right-skewed tree, k=4",
+			root: &TreeNode{
+				Val: 1,
+				Right: &TreeNode{
+					Val: 2,
+					Right: &TreeNode{
+						Val:   3,
+						Right: &TreeNode{Val: 4},
+					},
+				},
+			},
+			k:    4,
+			want: 4,
+		},
+		{
+			name: "Left-skewed tree, k=1",
+			root: &TreeNode{
+				Val: 4,
+				Left: &TreeNode{
+					Val: 3,
+					Left: &TreeNode{
+						Val:  2,
+						Left: &TreeNode{Val: 1},
+					},
+				},
+			},
+			k:    1,
+			want: 1,
+		},
+		{
+			name: "Left-skewed tree, k=3",
+			root: &TreeNode{
+				Val: 4,
+				Left: &TreeNode{
+					Val: 3,
+					Left: &TreeNode{
+						Val:  2,
+						Left: &TreeNode{Val: 1},
+					},
+				},
+			},
+			k:    3,
+			want: 3,
+		},
+		{
+			name: "Balanced tree, k=last element",
+			root: &TreeNode{
+				Val: 4,
+				Left: &TreeNode{
+					Val:   2,
+					Left:  &TreeNode{Val: 1},
+					Right: &TreeNode{Val: 3},
+				},
+				Right: &TreeNode{
+					Val:   6,
+					Left:  &TreeNode{Val: 5},
+					Right: &TreeNode{Val: 7},
+				},
+			},
+			k:    7,
+			want: 7,
+		},
+		{
+			name: "Balanced tree, k=middle element",
+			root: &TreeNode{
+				Val: 4,
+				Left: &TreeNode{
+					Val:   2,
+					Left:  &TreeNode{Val: 1},
+					Right: &TreeNode{Val: 3},
+				},
+				Right: &TreeNode{
+					Val:   6,
+					Left:  &TreeNode{Val: 5},
+					Right: &TreeNode{Val: 7},
+				},
+			},
+			k:    4,
+			want: 4,
+		},
+		{
+			name: "Large values",
+			root: &TreeNode{
+				Val:   10000,
+				Left:  &TreeNode{Val: 5000},
+				Right: &TreeNode{Val: 10004},
+			},
+			k:    2,
+			want: 10000,
+		},
+		{
+			name: "Values with 0",
+			root: &TreeNode{
+				Val:   1,
+				Left:  &TreeNode{Val: 0},
+				Right: &TreeNode{Val: 2},
+			},
+			k:    1,
+			want: 0,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := kthSmallest(tt.root, tt.k)
+			if got != tt.want {
+				t.Errorf("kthSmallest() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}