JAVA算法练习题day34

43.验证二叉搜索树

要知道二叉搜索树的中序遍历结果是升序序列

1# Definition for a binary tree node.
2# class TreeNode(object):
3#     def __init__(self, val=0, left=None, right=None):
4#         self.val = val
5#         self.left = left
6#         self.right = right
7class Solution(object):
8    def isValidBST(self, root):
9        """
10        :type root: Optional[TreeNode]
11        :rtype: bool
12        """
13        if root is None:
14            return True
15        if root.left is None and root.right is None:
16            return True
17        nums = []
18        def dfs(root):
19            if root is None:
20                return
21            dfs(root.left)
22            nums.append(root.val)
23            dfs(root.right)
24
25        dfs(root)
26        #写一个判断数组是否为升序数组的函数
27        def isST(nums):
28            for i in range(len(nums)-1):
29                if nums[i]>=nums[i+1]:
30                    return False
31            return True
32        return isST(nums)
33

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