144. 二叉树的前序遍历

题目描述

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吗？

我的解法——递归（AC）

# 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 getpre(self, tree):
        if(not tree):
            return []
        left = self.getpre(tree.left)
        right = self.getpre(tree.right)
        return [tree.val, ] + left + right
    
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        return self.getpre(root)
# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

我的解法——非递归（AC）

# 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 go_left(self, res, stack, now):
        while(now):
            res.append(now.val)
            stack.append(now)
            now = now.left
            
            
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        stack = []
        res = []
        now = root
        
        while(now):
            self.go_left(res, stack, now)
            
            while(True):
                if(not len(stack)):
                    now = None
                    break
                
                if(stack[-1].right):
                    now = stack[-1].right
                    stack = stack[: -1]
                    break
                else:
                    stack = stack[: -1]

        return res