Given a string s, return the number of distinct substrings of s. A substring of a string is obtained by deleting any number of characters (possibly zero) from the front of the string and any number (possibly zero) from the back of the string. Example 1: Input: s = "aabbaba" Output: 21 Explanation: The set of distinct strings is ["a","b","aa","bb","ab","ba","aab","abb","bab","bba","aba","aabb","abba","bbab","baba","aabba","abbab","bbaba","aabbab","abbaba","aabbaba"] Example 2: Input: s = "abcdefg" Output: 28 Constraints: 1 <= s.length <= 500 s consists of lowercase English letters. Follow up: Can you solve this problem in O(n) time complexity?