Thread: program taking too long to complete

i'am trying to solve a problem:

Given a string of N 0's and M 1's,how many unique permutations of this string start with 1?

answers for each test case is printed modulo(10^9 +7).

however my program takes too long to complete.
please suggest a much faster method.

Code:

#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
int main()
{
    int T;
    cin>>T;
    string s;
    int z,o;
    long long ans;
    while(T--)
    {
        s="";
        ans=1;
        cin>>z>>o;
        for(auto i=0;i<o;++i)
            s+='1';
        for(auto i=0;i<z;++i)
            s+='0';
        while(prev_permutation(s.begin(),s.end()))
        {
            if(s[0]=='0')
                break;
            ++ans;
        }
        cout<<ans%(1000000007)<<'\n';
    }
}