Merge pull request neetcode-gh#1037 from julienChemillier/patch-6

Add 98 in c language

Author: Ahmad-A0

c/98-Validate-Binary-Search-Tree.c

@@ -0,0 +1,18 @@
+/*
+Given the root of a binary tree, determine if it is a valid binary search tree (BST).
+Time: O(n)
+Space: O(1)
+*/
+
+#define MAX 9223372036854775807
+#define MIN -9223372036854775808
+
+bool isSubTreeValid(struct TreeNode* root, long int lowerBound, long int upperBound) {
+    if (root==NULL)
+        return true;
+    return root->val<upperBound && root->val>lowerBound && isSubTreeValid(root->left, lowerBound, root->val) && isSubTreeValid(root->right, root->val, upperBound);
+}
+
+bool isValidBST(struct TreeNode* root){
+    return isSubTreeValid(root, MIN, MAX);
+}