Validate Binary Search Tree

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.

思路

递归，对于每个根节点，判断左右子节点是否在规定的大小区间内，然后再判断两个子树是否是合法的。

    bool isValid(TreeNode *root, int min, int max){
        if(root == NULL)
            return true;
        if(root->val <= min || root->val >= max)
            return false;
        return isValid(root->left, min, root->val)&&isValid(root->right, root->val, max);
    }
    bool isValidBST(TreeNode *root) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        if(root == NULL)
            return true;
        return isValid(root->left, INT_MIN, root->val)&&isValid(root->right, root->val, INT_MAX);
    }