题目链接：https://leetcode.com/problems/binary-tree-preorder-traversal/

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?

思路：

题目要求用迭代，那么用 栈来实现

AC 1ms Java：

/**
 * 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> ans=new ArrayList();
        if(root==null)
            return ans;
        Stack<TreeNode> sk=new Stack();
        sk.push(root);
        while(!sk.isEmpty()){
            TreeNode temp=sk.pop();
            ans.add(temp.val);
            if(temp.right!=null)
                sk.push(temp.right);
            if(temp.left!=null)
                sk.push(temp.left);
        }
        return ans;
    }
}