# Distinct Subsequences II Problem

## Description

LeetCode Problem 940.

Given a string s, return the number of distinct non-empty subsequences of s. Since the answer may be very large, return it modulo 10^9 + 7.

A subsequence of a string is a new string that is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (i.e., “ace” is a subsequence of “abcde” while “aec” is not.

Example 1:

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Input: s = "abc"
Output: 7
Explanation: The 7 distinct subsequences are "a", "b", "c", "ab", "ac", "bc", and "abc".

Example 2:

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Input: s = "aba"
Output: 6
Explanation: The 6 distinct subsequences are "a", "b", "ab", "aa", "ba", and "aba".

Example 3:

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Input: s = "aaa"
Output: 3
Explanation: The 3 distinct subsequences are "a", "aa" and "aaa".

Constraints:

- 1 <= s.length <= 2000
- s consists of lowercase English letters.

## Sample C++ Code

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class Solution {
public:
int distinctSubseqII(string s) {
int mod = 1e9 + 7;
int n = s.size();
vector<long long int > dp(n+1, 0);
dp[0] = 1;
map<char,int> m;
for (int i = 1; i <= n ;i++) {
dp[i] = (2 * dp[i-1]) % mod;
char a = s[i-1];
if (m.count(a)) {
int j = m[a];
dp[i] = (dp[i] - dp[j-1]) % mod;
}
m[a] = i;
}
if (dp[n] < 0)
dp[n] += mod;
return dp[n] - 1;
}
};