Added Valid Parenthesis String.java

Author: varunu28

Medium/Valid Parenthesis String.java

@@ -0,0 +1,15 @@
+class Solution {
+  public boolean checkValidString(String s) {
+    int low = 0;
+    int high = 0;
+    for (char c : s.toCharArray()) {
+      low += c == '(' ? 1 : -1;
+      high += c != ')' ? 1 : -1;
+      if (high < 0) {
+        break;
+      }
+      low = Math.max(low, 0);
+    }
+    return low == 0;
+  }
+}