验证二叉搜索树

两种写法

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
# 思路：
# 1、中序遍历，将结果保存在ret中；
# 2、遍历ret，看是否是单增；
class Solution:
    def isValidBST(self, root: TreeNode) -> bool:
        if not root:
            return True
        tmp = root
        stack = []
        ret = []
        while tmp or stack:
            while tmp:
                stack.append(tmp)
                tmp = tmp.left
            node = stack.pop()
            ret.append(node.val)
            tmp = node.right
        
        p1 = 0
        p2 = 1
        while p2 < len(ret):
            if ret[p1] >= ret[p2]:
                return False
            p1 += 1
            p2 += 1
        return True

class Solution:
    def isValidBST(self, root: TreeNode) -> bool:
        res = []
        self.search(root, res)
        for i in range(1,len(res)):
            if res[i] <= res[i-1]:
                return False
        return True

    def search(self, root, res):
        if root:
            self.search(root.left, res)
            res.append(root.val)
            self.search(root.right, res)