Codeforces 1188B Count Pairs

#include<bits/stdc++.h>
#define LL long long
#define LD long double
#define ull unsigned long long
#define fi first
#define se second
#define mk make_pair
#define PLL pair<LL, LL>
#define PLI pair<LL, int>
#define PII pair<int, int>
#define SZ(x) ((int)x.size())
#define ALL(x) (x).begin(), (x).end()
#define fio ios::sync_with_stdio(false); cin.tie(0);
 
using namespace std;
 
const int N = 3e5 + 7;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 998244353;
const double eps = 1e-8;
const double PI = acos(-1);
 
template<class T, class S> inline void add(T &a, S b) {a += b; if(a >= mod) a -= mod;}
template<class T, class S> inline void sub(T &a, S b) {a -= b; if(a < 0) a += mod;}
template<class T, class S> inline bool chkmax(T &a, S b) {return a < b ? a = b, true : false;}
template<class T, class S> inline bool chkmin(T &a, S b) {return a > b ? a = b, true : false;}
 
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
 
LL n, p, k;
LL a[N];
LL hs[N], tot;
LL c[N];
map<LL, LL> Map;
 
int main() {
    scanf("%lld%lld%lld", &n, &p, &k);
    for(int i = 1; i <= n; i++) scanf("%lld", &a[i]);
    for(int i = 1; i <= n; i++) {
        hs[++tot] = a[i];
    }
    sort(hs + 1, hs + 1 + tot);
    tot = unique(hs + 1, hs + 1 + tot) - hs - 1;
    for(int i = 1; i <= n; i++) {
        c[lower_bound(hs + 1, hs + 1 + tot, a[i]) - hs]++;
    }
    LL ans = 0;
    for(int i = 1; i <= tot; i++) {
        LL v = hs[i];
        LL tmp = (v * v % p * v % p * v % p - k * v % p + p) % p;
        ans += Map[tmp];
        LL tmp2 = 2 * v % p * ((v * v % p + v * v % p) % p) % p;
        if(tmp2 == k) ans += c[i] * (c[i] - 1) / 2;
        Map[tmp] += c[i];
    }
    printf("%lld
", ans);
    return 0;
}
 
/*
*/