feat(Code Challenge: hasDuplicate()): finish

src/main/java/DuplicateCheck.java

@@ -0,0 +1,24 @@
+
+public class DuplicateCheck {
+    public static boolean hasDuplicate(String s) {
+        for (int i = 0; i < s.length(); i++) {
+            String curLetter = s.substring(i, i + 1);
+            for (int j = i + 1; j < s.length(); j++) {
+                if (curLetter.equals(s.substring(j, j + 1))) {
+                    return true;
+                }
+            }
+        }
+
+        return false;
+    }
+
+    public static void main(String[] args) {
+        System.out.println(hasDuplicate("null")); // true
+        System.out.println(hasDuplicate("abc")); // false
+        System.out.println(hasDuplicate("abca")); // true
+        System.out.println(hasDuplicate("llun")); // true
+        String funstr = "~!@#$%^&*()_+`1234567890-=qwertyuiop[]\"QWERTYUIOP{}asdfghjkl;'ASDFGHJKL:zxcvbnm,./ZXCVBNM<>?";
+        System.out.println(hasDuplicate(funstr)); // false
+    }
+}