[Leetcode] 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?

[Thoughts]

常见面试题，主要考两个东西：

　　1.什么是先序遍历

　　2.Java中Stack的使用

[Code]

/**
 * Definition for binary tree
 * 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> result = new ArrayList<Integer>();
        
        if(root == null)
            return result;
        
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode temp = stack.pop();
            result.add(temp.val);
            
            if(temp.right != null){
                stack.push(temp.right);
            }
            if(temp.left != null){
                stack.push(temp.left);
            }
        }
        return result;
    }
}