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Program to count number of unique subsequences of a string in C++
Suppose we have a string s, we have to find the number of non-empty unique subsequences of s. If the answer is very large then mod the result by 10^9 + 7.
So, if the input is like s = "xxy", then the output will be 5, as there are five subsequences: "x", "xx", "xy", "y" and "xxy".
To solve this, we will follow these steps −
m := 10^9 + 7
n := size of s
Define an array table of size 26
res := 0
for initialize i := 1, when i <= n, update (increase i by 1), do−
c := s[i − 1] − ASCII of 'a'
curr := (res + 1 − table[c] + m) mod m
res := (res + curr) mod m
table[c] := (table[c] + curr) mod m
return res
Let us see the following implementation to get better understanding −
Example
#include <bits/stdc++.h>
using namespace std;
const int m = 1e9 + 7;
int solve(string s) {
int n = s.size();
vector<int> table(26);
long long res = 0;
for (int i = 1; i <= n; ++i) {
int c = s[i − 1] − 'a';
int curr = (res + 1 − table[c] + m) % m;
res = (res + curr) % m;
table[c] = (table[c] + curr) % m;
}
return res;
}
int main(){
string s = "xxy";
cout << solve(s);
}Input
"xxy"
Output
5
Updated on: 26-Dec-2020
501 Views
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