Given a string,
Lisa reads it from left to right, and She stopes at the first repeated subString
Please write a function to help her to find the first repeated subString in it. Return "" if there's no repeated subString
Example: inputString: "fabcdeabfa" Expected Output: "ab" (if there're many subStrings repeated in the same first index like "a" and "ac" of "dacbacda", return the longest one "ac")
