Binary Tree Preorder Traversal

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

For example: Given binary tree {1,#,2,3},

1 \ 2 / 3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ret = new ArrayList<Integer>();
        if(root == null) return ret;

        Stack<TreeNode> stack = new Stack<TreeNode>();

        stack.push(root);

        while(!stack.isEmpty()) {
            TreeNode node = stack.pop();
            ret.add(node.val);

            if(node.right != null) {
                stack.push(node.right);
            }

            if(node.left != null) {
                stack.push(node.left);
            }
        }

        return ret;

    }
}