145.py

# leetcode 145 二叉树的后序遍历

## 第一种 递归法

# 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]:
        if not root:
            return []

        right = self.postorderTraversal(root.right)
        left = self.postorderTraversal(root.left)

        return left + right + [root.val]


# 第二种 迭代法

# 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]:

        if not root:
            return []

        res = []
        stack = [root]

        while stack:
            tmp = stack.pop()
            res.append(tmp.val)
            if tmp.left:
                stack.append(tmp.left)
            if tmp.right:
                stack.append(tmp.right)

        return list(reversed(res))