和先序遍历类似,但入栈顺序是先push左节点,再push右节点,最后将结果倒置
# 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: TreeNode) -> List[int]:
ans = []
if not root: return ans
stack = [root]
while stack:
tmp = stack.pop()
ans.append(tmp.val)
if tmp.left: stack.append(tmp.left)
if tmp.right: stack.append(tmp.right)
return ans[::-1]