leetcode-145-二叉树的后序遍历

题目描述：

方法一：递归

class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        def helper(root):
            if not root:
                return 
            helper(root.left)
            helper(root.right)
            res.append(root.val)
        helper(root)
        return res

方法二：迭代

class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        if not root:
            return res
        stack = [root]
        while stack:
            node = stack.pop()
            if node.left:
                stack.append(node.left)
            if node.right:
                stack.append(node.right)
            res.append(node.val)
        return res[::-1]
            

 另：

class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        if not root: return []
        stack = []
        p = root
        res = []
        flag = []
        while p or stack:
            while p:
                flag.append(0)
                stack.append(p)
                p = p.left
            while stack and flag[-1] == 1:
                flag.pop()
                res.append(stack.pop().val)
            if stack:
                flag[-1] = 1
                p = stack[-1].right
        return res