leetcode 98. 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.

Example 1:

    2   / \  1   3

Binary tree [2,1,3], return true.

Example 2:

    1   / \  2   3

Binary tree [1,2,3], return false.

使用一个递归的思想。当前节点成立的条件就是子节点也得成立。

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    bool check(TreeNode *p, int min, int max)    {        if (!p)            return 1;        else if (p->val > max || p->val < min)              return 0;                  TreeNode *l = p->left;        TreeNode *r = p->right;                if ( (l && p->val <= l->val) || (r && p->val >= r->val) )  //提前比较，当前节点可能是INT_MIN,左子节点就无法再减1，会出现未知错误            return 0;            return (check(l, min, p->val - 1) && check(r, p->val + 1, max));    }        bool isValidBST(TreeNode* root)     {        return check( root, INT_MIN, INT_MAX );        } };