Given a string, find the length of the longest substring without repeating characters.
 *  For example, the longest substrings without repeating characters for “ABDEFGABEF” are “BDEFGA” and “DEFGAB”, with length 6.
 *  For “BBBB” the longest substring is “B”, with length 1.
 * For “GEEKSFORGEEKS”, there are two longest substrings shown in the below diagrams, with length 7.