add stringUtil

src/main/java/utils/StringUtil.java

@@ -0,0 +1,19 @@
+package utils;
+
+public class StringUtil {
+
+    // https://stackoverflow.com/questions/2255500/can-i-multiply-strings-in-java-to-repeat-sequence
+    private String multiplyStr(String s, int multiply){
+        StringBuilder r = new StringBuilder();
+        for (int i = 0; i < multiply; i++){
+            r.append(s);
+        }
+        return r.toString();
+    }
+
+//    public static void main(String[] args) {
+//        String res = multiplyStr("0", 3);
+//        System.out.println(res);
+//    }
+
+}