luogu P3799 妖梦拼木棒

二次联通门 : luogu P3799 妖梦拼木棒

/*
    luogu P3799 妖梦拼木棒

    用一个桶存下所有的木棒

    美剧两根短的木棒长度

    后随便乘一乘就 好了。。 

*/
#include <algorithm>
#include <cstdio>

#define Mod 1000000007

#define Max 5000

void read (int &now)
{
    now = ;
    register char word = getchar ();
    while (word < '' || word > '')
        word = getchar ();
    while (word >= '' && word <= '')
    {
        now = now *  + word - '';
        word = getchar ();
    }
}

int N;

long long count[Max << ];
long long Answer;

int main (int argc, char *argv[])
{
    read (N);
    int x;
    for (register int i = ; i <= N; i ++)
    {
        read (x);
        count[x] ++;
    }

    for (register int i = ; i <= Max; i ++)
        for (register int j = i; j <= Max; j ++)
        {
            if (i + j > Max)
                break;
            if (i == j)
            {
                if (count[i] >=  && count[i << ] >= )
                    Answer = (Answer + count[i] * (count[i] - ) / 2LL % Mod * (count[i << ] * (count[i << ] - ) /  % Mod)) % Mod;
            }
            else
                if (count[i] >=  && count[j] >=  && count[i + j] >= )
                    Answer = (Answer + (count[i] * count[j] % Mod * (count[i + j] * (count[i + j] - ) / )% Mod)) % Mod;

        }

    printf ("%lld", Answer);
    return ;
}