## Validate Binary Search Tree

2018年04月12日 ⁄ 综合 ⁄ 共 789字 ⁄ 字号 评论关闭

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.

```/**
* 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 isValidBST(TreeNode *root) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if ( !root )
return true;
pre=-1000000;
return check(root);
}
bool check(TreeNode* root)
{
if ( !root )
return true;
if ( check(root->left) )
{
if ( root->val <= pre)
return false;
pre=root->val;
return check(root->right);
}
else
return false;

}
int pre;
};```