Binary Tree Preorder Traversal

BinaryTreePreorderTraversal.java

/**
 * 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?
 */
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
 
 public class Solution {
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> res = new ArrayList<Integer>();
        if(root == null)
            return res;
        Stack<TreeNode> s = new Stack<TreeNode>();
        s.push(root);
        while(!s.isEmpty()){
            TreeNode tmp = s.pop();
            res.add(tmp.val);
            // pay more attention to this sequence. 
            if(tmp.right != null)
                s.push(tmp.right);
            if(tmp.left != null)
                s.push(tmp.left);  
        }
        return res;
    }
 }