leetcode 144 二叉树的前序遍历

题目

给定一个二叉树，返回它的 前序 遍历。

 示例:

输入: [1,null,2,3]  
   1
    \
     2
    /
   3 

输出: [1,2,3]

进阶: 递归算法很简单，你可以通过迭代算法完成

代码

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root == None:
            return
        res = []
        s = []
        cur = root
        while (len(s) != 0) | (cur != None):
            while cur:
                res.append(cur.val)
                s.append(cur)
                cur = cur.left
            cur = s.pop()
            cur = cur.right
        return res