数学--数论--HDU 6128 Inverse of sum （公式推导论）

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define _m(a, p) make_pair(a, p)
map<ll, ll> mp;
map<ll, ll> mm;
ll mod;
long long ksc(long long a, long long b, long long mod)
{
    long long ans = 0;
    for (; b; b >>= 1){
        if (b & 1)
            ans = (ans + a) % mod;
            a = (a + a) % mod; //（计算机加法比乘法快，a+a比a*2快）
    }
    return ans;
}
int main()
{
    int t, n;
    scanf("%d", &t);
    while (t--)
    {
        ll cnt = 0;
        mp.clear();
        mm.clear();
        scanf("%d %lld", &n, &mod);
        for (int i = 0; i < n; i++)
        {
            ll a;
            scanf("%lld", &a);
            if (!a)
                continue;
            mm[a]++;
            ll ans = ksc(ksc(a, a, mod), a, mod);
            mp[ans]++;
        }
        for (auto p : mp)
        {
            ll cc = p.second;
            cnt += cc * (cc - 1) / 2;
        }
        if (mod != 3)
            for (auto m : mm)
            {
                ll n = m.second;
                cnt -= n * (n - 1) / 2;
            }
        printf("%lld\n", cnt);
    }
    return 0;
}