LeetCode-Binary Tree Preorder Traversal

题目：https://oj.leetcode.com/problems/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?

源码：Java版本
算法分析：用栈。时间复杂度O(n)，空间复杂度O(n)

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