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LeetCode: Validate Binary Search Tree [098]
【题目】
Given a binary tree, determine if it is a valid binary search tree (BST).
Assume a BST is defined as follows:
- The left subtree of a node contains only nodes with keys less than the node‘s key.
- The right subtree of a node contains only nodes with keys greater than the node‘s key.
- Both the left and right subtrees must also be binary search trees.
confused what "{1,#,2,3}"
means? > read more on how binary tree is serialized on OJ.
【题意】
给定一棵二叉树,判断是不是合法的二叉搜索树【思路】
根据二叉搜索树定义,递归判断即可【代码】
/** * Definition for binary tree * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */ class Solution { public: bool isValid(TreeNode*root, int lowBound, int upBound){ //每棵树取值都有上边界和下边界 if(root==NULL)return true; //判断节点值是否在合法的取值区间内 if(!(root->val>lowBound && root->val<upBound))return false; //判断左子树是否合法 if(root->left){ if(root->left->val >= root->val || !isValid(root->left, lowBound, root->val))return false; } //判断右子树 if(root->right){ if(root->right->val <= root->val || !isValid(root->right, root->val, upBound))return false; } return true; } bool isValidBST(TreeNode *root) { return isValid(root, INT_MIN, INT_MAX); } };
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