Leetcode 98.验证二叉搜索树

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方法

方法1：递归(中序遍历)

中序遍历二叉搜索树，一般可以得到递增的中序序列

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# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if not root:
            return True

        output=[]
        self.inorder(root,output)

        for i in range(1,len(output)):
            if output[i]<=output[i-1]:
                return False
        return True

    def inorder(self,root,output):
        if not root:
            return
        self.inorder(root.left,output)
        output.append(root.val)
        self.inorder(root.right,output)

方法2：非递归中序遍历

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# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        stack=[]
        ans=[]
        while root or stack:
            while root:
                stack.append(root)
                root=root.left
            root=stack.pop()
            ans.append(root.val)
            if len(ans)>=2 and ans[-1]<=ans[-2]:
                return False
            root=root.right
        return True