BinaryTreePreorderTraversalIterative.java

/*
Given a binary tree, return the preorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]
Follow up: Recursive solution is trivial, could you do it iteratively?
*/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) 
    {
        List<Integer> tgt = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        
        while (root != null || !stack.isEmpty())
        {
            if (root != null)
            {
                tgt.add(root.val);
                stack.push(root);
                root = root.left;
            }
            else
            {
                root = stack.pop();
                root = root.right;
            }
        }
        
        return tgt;
    }
}