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:
<li><code>S</code> contains only lowercase letters.</li>
<li><code>1 <= S.length <= 2000</code></li>