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Distinct Subsequences II in C++
Suppose we have a string S, we have to count the number of distinct subsequences of S. The result can be large, so we will return the answer modulo 10^9 + 7.
So, if the input is like "bab", then the output will be 6, as there are 6 different sequences, these are "a", "b, "ba", "ab", "bb", "abb".
To solve this, we will follow these steps −
Define a function add(), this will take a, b,
return ((a mod MOD) + (b mod MOD)) mod MOD
Define a function sub(), this will take a, b,
return (((a mod MOD) - (b mod MOD)) + MOD) mod MOD
Define a function mul(), this will take a, b,
return ((a mod MOD) * (b mod MOD)) mod MOD
From the main method, so the following −
n := size of s
Define an array dp of size 26
res := 0
s := concatenate space before s
for initialize i := 1, when i <= n, update (increase i by 1), do −
x := s[i]
added := sub(add(res, 1), dp[x - 'a'])
dp[x - 'a'] = add(dp[x - 'a'], added)
res := add(res, added)
return res
Let us see the following implementation to get better understanding −
Example
#include <bits/stdc++.h>
using namespace std;
typedef long long int lli;
const lli MOD = 1e9 + 7;
class Solution {
public:
lli add(lli a, lli b){
return ( (a % MOD) + (b % MOD) ) % MOD;
}
lli sub(lli a, lli b){
return ( ( (a % MOD) - (b % MOD) ) + MOD ) % MOD;
}
lli mul(lli a, lli b){
return ( (a % MOD) * (b % MOD) ) % MOD;
}
int distinctSubseqII(string s) {
int n = s.size();
vector <lli> dp(26);
int res = 0;
s = " " + s;
for(lli i = 1; i <= n; i++){
char x = s[i];
int added = sub(add(res, 1) , dp[x - 'a']);
dp[x - 'a'] = add(dp[x - 'a'], added);
res = add(res, added);
}
return res;
}
};
main(){
Solution ob;
cout << (ob.distinctSubseqII("bab"));
}Input
"bab"
Output
6
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