LeetCode145-二叉树的后序遍历

原题链接：https://leetcode-cn.com/problems/binary-tree-postorder-traversal/

解法一：递归，需了解树的递归式遍历

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def postorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        def postorder(root: TreeNode):
            if not root:
                return
            postorder(root.left)
            postorder(root.right)
            res.append(root.val)
        res = list()
        postorder(root)
        return res

 

解法二：采用辅助栈

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
from collections import deque
class Solution:
    def postorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        # 需要创建一个中转栈，即为收集栈
        # 1、每次弹出当前栈，即为cur
        # 2、cur放入收集栈
        # 3、先左再右入栈
        # 4、周而复始
        # 5、弹出收集栈
        res = list()
        if root:
            s1 = deque()
            s2 = deque()
            s1.append(root)
            while s1:
                cur = s1.pop()
                s2.append(cur)
                if cur.left:
                    s1.append(cur.left)
                if cur.right:
                    s1.append(cur.right)
            while s2:
                res.append(s2.pop().val)
        return res