【leetcode 145】二叉樹的後序遍歷

和先序遍歷類似，但入棧順序是先push左節點，再push右節點，最後將結果倒置

# 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]