# 940. Distinct Subsequences II

## Description

Given a string s, return the number of distinct non-empty subsequences of s. Since the answer may be very large, return it modulo 109 + 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:

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


Example 2:

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


Example 3:

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.

## Solutions

• class Solution {
private static final int MOD = (int) 1e9 + 7;

public int distinctSubseqII(String s) {
int[] dp = new int[26];
int ans = 0;
for (int i = 0; i < s.length(); ++i) {
int j = s.charAt(i) - 'a';
int add = (ans - dp[j] + 1) % MOD;
ans = (ans + add) % MOD;
dp[j] = (dp[j] + add) % MOD;
}
return (ans + MOD) % MOD;
}
}

• class Solution {
public:
const int mod = 1e9 + 7;

int distinctSubseqII(string s) {
vector<long> dp(26);
long ans = 0;
for (char& c : s) {
int i = c - 'a';
long add = ans - dp[i] + 1;
ans = (ans + add + mod) % mod;
dp[i] = (dp[i] + add) % mod;
}
return ans;
}
};

• class Solution:
def distinctSubseqII(self, s: str) -> int:
mod = 10**9 + 7
dp = [0] * 26
ans = 0
for c in s:
i = ord(c) - ord('a')
add = ans - dp[i] + 1
ans = (ans + add) % mod
return ans


• func distinctSubseqII(s string) int {
const mod int = 1e9 + 7
dp := make([]int, 26)
ans := 0
for _, c := range s {
c -= 'a'
add := ans - dp[c] + 1
ans = (ans + add) % mod
dp[c] = (dp[c] + add) % mod
}
return (ans + mod) % mod
}

• function distinctSubseqII(s: string): number {
const mod = 1e9 + 7;
const dp = new Array(26).fill(0);
for (const c of s) {
dp[c.charCodeAt(0) - 'a'.charCodeAt(0)] = dp.reduce((r, v) => (r + v) % mod, 0) + 1;
}
return dp.reduce((r, v) => (r + v) % mod, 0);
}


• impl Solution {
pub fn distinct_subseq_ii(s: String) -> i32 {
const MOD: i32 = (1e9 as i32) + 7;
let mut dp = [0; 26];
for u in s.as_bytes() {
let i = (u - &b'a') as usize;
dp[i] =
({
let mut sum = 0;
dp.iter().for_each(|&v| {
sum = (sum + v) % MOD;
});
sum
}) + 1;
}
let mut res = 0;
dp.iter().for_each(|&v| {
res = (res + v) % MOD;
});
res
}
}