145. 二叉树的后序遍历

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]:res = []def helper(root, res):if root:helper(root.left, res)helper(root.right, res)res.append(root.val)helper(root, res)return res

迭代法

先序遍历是中左右，后续遍历是左右中，那么我们只需要调整一下先序遍历的代码顺序，就变成中右左的遍历顺序，然后在反转res数组，输出的结果顺序就是左右中了

# 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 []stack, res = [root], []while stack:node = stack.pop()res.append(node.val)if node.left:stack.append(node.left)if node.right:stack.append(node.right)return res[::-1]