题目描述(Medium)
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.题目链接
https://leetcode.com/problems/validate-binary-search-tree/description/
Example 1:
Input: 2 / \ 1 3 Output: true
Example 2:
5 / \ 1 4 / \ 3 6 Output: false Explanation: The input is: [5,1,4,null,null,3,6]. The root node's value is 5 but its right child's value is 4.
算法分析
中序遍历后判断序列的大小关系或者直接遍历，判断根节点与左右子树的关系。
提交代码：
class Solution { public: bool isValidBST(TreeNode* root) { return isValidBST(root, LONG_MIN, LONG_MAX); } bool isValidBST(TreeNode* root, long lower, long upper) { if (root == nullptr) return true; return root->val > lower && root->val < upper && isValidBST(root->left, lower, root->val) && isValidBST(root->right, root->val, upper); } };
