Spoj Subset Sums

#include <algorithm>
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <ctime>

using namespace std;
typedef long long LL;
#define FOR(k,a,b) for(int k(a); k < (b); ++k)
#define REP(k,a) for(int k=0; k < (a); ++k)

int main()
{
    int N,A,B;
    cin >> N >> A >> B;
    vector<LL> S(N);
    LL res=0;
    REP(i,N)
    {
        cin >> S[i] ;
    }
    vector<LL> v;
    int vi=N/2,wi=N-vi;
    REP(i,1<<vi)
    {
        v.push_back(0);
        REP(j,vi)
        {
            if((1<<j) &i )
                v.back()+=S[j];
        }
        if(v.back()>=A && v.back()<=B)
            ++res;
    }
    sort(v.begin(),v.end());
    FOR(i,1,1<<wi)
    {
        LL w=0;
        REP(j,wi)
        {
            if((1<<j) &i )
                w+=S[j+vi];
        }
        //binary search
        //w+v_i>=A => min   i: v_i>=A-w
        //w+v_i<=B => max   i: v_i<=B-w
        w=A-w;
        vector<LL>::iterator low=lower_bound(v.begin(),v.end(),w);
        w=B-A+w;
        LL dist=0;
        vector<LL>::iterator up=upper_bound(v.begin(),v.end(),w);
        dist=distance(low,up);
        res+=dist;
    }
    cout << res << endl;
    return 0;
}