Given a binary tree, determine if it is a valid binary search tree (BST).
Assume a BST is defined as follows:
Example 1:
2 / 1 3
Binary tree [2,1,3], return true.
Example 2:
1 / 2 3
Binary tree [1,2,3], return false.
对每一个节点使用一个upper bound and lower bound. 时间复杂度 O(N).
1 def isValidBST(self, root): 2 """ 3 :type root: TreeNode 4 :rtype: bool 5 """ 6 maxInt = 2147483647 7 minInt = -2147483648 8 9 return self.isBSTHelper(root, minInt, maxInt) 10 11 def isBSTHelper(self, root, minVal, maxVal): 12 if not root: 13 return True 14 15 if root.val <= maxVal and root.val >= minVal and self.isBSTHelper(root.left, minVal, root.val - 1) and self.isBSTHelper(root.right, root.val + 1, maxVal): 16 return True 17 else: 18 return False
Leetcode 98. Validate Binary Search Tree
原文：http://www.cnblogs.com/lettuan/p/6196402.html