Fix: validate bin search tree

Author: dylan-jung

validate-binary-search-tree/dylan-jung.cpp

@@ -9,21 +9,22 @@
  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
  * };
  */
- class Solution {
+class Solution {
 public:
     bool dfs(TreeNode* root, long minVal, long maxVal) {
         if(!(minVal < root->val && root->val < maxVal)) return false;
         bool isValid = true;
-        if(root->left) {
-            isValid = isValid && dfs(root->left, minVal, min((long)root->val, maxVal));
+        if (root->left) {
+            isValid = isValid && dfs(root->left, minVal, root->val);
         }
-        if(root->right) {
-            isValid = isValid && dfs(root->right, max((long)root->val, minVal), maxVal);
+        if (root->right) {
+            isValid = isValid && dfs(root->right, root->val, maxVal);
         }
         return isValid;
     }
 
     bool isValidBST(TreeNode* root) {
+        if(!root) return true;
         return dfs(root, -(1l << 32), 1l << 32);
     }
 };