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:
      void  isValid(TreeNode *root ,int &pre,bool &s) {
          if(root==NULL)
          return ;
          else{
           
            isValid(root->left,pre ,s);
          
            if(root->val <= pre ){
                s = false;
                return;
            }
            pre = root->val;
            isValid(root->right,pre,s);
            
            
            
          }
          
      }
      bool isValidBST(TreeNode *root) {
          bool s=true;
          int pre=-2000000;
           isValid(root,pre,s);
          return s;
      }
  };