[LeetCode] Validate Binary Search Tree

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.

/**
 * 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
        
    	int pre=INT_MIN;
		return inorder(root,pre);
    }
	bool inorder(TreeNode* root,int& pre)
	{
		if (!root )
			return true;
		if (!inorder(root->left,pre))
			return false;
		if ( pre>=root->val)
			return false;
		pre=root->val;
		return inorder(root->right,pre);
	}
};