Create 678. Valid Parenthesis String (#450)

Author: Chayandas07

678. Valid Parenthesis String

@@ -0,0 +1,19 @@
+#include <iostream>
+#include <string>
+#include <algorithm>
+
+using namespace std;
+
+class Solution {
+public:
+    bool checkValidString(string s) {
+        int low = 0, high = 0;
+        for (char c : s) {
+            low += (c == '(') ? 1 : -1;
+            high += (c != ')') ? 1 : -1;
+            if (high < 0) return false;
+            low = max(low, 0);
+        }
+        return low == 0;
+    }
+};