Given a string S, count the number of distinct, non-empty subsequences of S .
Since the result may be large, return the answer modulo 10^9 + 7.
Example 1:
Input: "abc"
Output: 7
Explanation: The 7 distinct subsequences are "a", "b", "c", "ab", "ac", "bc", and "abc".
Example 2:
Input: "aba"
Output: 6
Explanation: The 6 distinct subsequences are "a", "b", "ab", "ba", "aa" and "aba".
Example 3:
Input: "aaa"
Output: 3
Explanation: The 3 distinct subsequences are "a", "aa" and "aaa".
Note:
S contains only lowercase letters.1 <= S.length <= 2000