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

class Solution {
public:
    bool f(TreeNode * root,int low,int high){
        if (!root){
            return true;
        }
        return low < root->val && root->val < high && f(root->left,low,root->val) && f(root->right,root->val,high);
    }
    bool isValidBST(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int low = INT_MIN, high = INT_MAX;
        return f(root,low,high);
    }
};