Update validate_binary_search_tree.cpp

Modify solution to be more similar to Neetcode's solution.

cpp/neetcode_150/07_trees/validate_binary_search_tree.cpp

@@ -21,29 +21,20 @@
 class Solution {
 public:
  bool isValidBST(TreeNode* root) {
- TreeNode* prev = NULL;
- return inorder(root, prev);
- }
-private:
- bool inorder(TreeNode* root, TreeNode*& prev) {
- if (root == NULL) {
- return true;
- }
-
- if (!inorder(root->left, prev)) {
- return false;
- }
-
- if (prev != NULL && prev->val >= root->val) {
- return false;
- }
- prev = root;
+ using def = function<bool(TreeNode*,long,long)>;
 
- if (!inorder(root->right, prev)) {
- return false;
- }
+ def valid = [&] (auto node, long left, long right) {
+ if(!node){
+ return true; 
+ }
+ if(!(node->val < right && node->val > left)){
+ return false;
+ }
+ return (valid(node->left, left, node->val) && 
+ valid(node->right, node->val, right));
+ };
 
- return true;
+ return valid(root, long(INT_MIN)-1, long(INT_MAX)+1);
  }
 };