题解 | #二叉树的后序遍历#

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param root TreeNode类 
# @return int整型一维数组
#
class Solution:
    def postorderTraversal(self , root ):
        res = []
        self.post_order(root, res)
        return res
    
    def pre_order(self,root,res):
        if not root :
            return
        stack = [root]
        while stack:
            top = stack.pop()
            res.append(top.val)
            if top.right:
                stack.append(top.right)
            if top.left:
                stack.append(top.left)
                
    def in_order(self,root,res):
        stack = []
        while root:
            stack.append(root)
            root = root.left
        while stack:
            top = stack.pop()
            res.append(top.val)
            right = top.right
            while right:
                stack.append(right)
                right = right.left
        return res
            
    def post_order(self,root,res):
        if not root:
            return
        stack = [root]
        pre_node = None
        while stack:
            top = stack[-1]
            if (not top.left and not top.right) or \
               (top.right and pre_node == top.right) or \
               (not top.right and pre_node==top.left):
                stack.pop()
                pre_node = top
                res.append(top.val)
            else:
                if top.right:
                    stack.append(top.right)
                if top.left:
                    stack.append(top.left)
        return res