2019 icpc ecfinal H.King 思维 模拟 随机数

一个思维题，切入点是n/2

写代码后交疯狂wa18，然后发现rand函数的上限是32767，太小了，换了一个随机数就过了。

#include <bits/stdc++.h>
using namespace std;
#define ll long long
const int N = 200020;

int n, mod;

std::mt19937 rnd(time(0));
int a[N];

ll qmi(ll a, int k) {
    ll res = 1;
    while(k) {
        if(k & 1) res = (res * a) % mod;
        a = (a * a) % mod;
        k >>= 1;
    }
    return res;
}

int main() {
    int t;
    scanf("%d", &t);
    while(t --) {
        scanf("%d%d", &n,&mod);
        for(int i = 1;i <= n;i ++) scanf("%d", &a[i]);

        int ans = 0;
        for(int Case = 1;Case <= 500;Case ++) {
            ll x = rnd() % n + 1;

            for(ll l = 1;l <=2 && l + x <= n;l ++) {
                int num = 2;
                ll q = (a[l+x] * qmi(a[x], mod-2)) % mod;
                ll last = (a[l+x] * q) % mod;
                for(int i = l + x + 1;i <= n;i ++) {
                    if(a[i] == last) {
                        num ++;
                        last = (last * q) % mod;
                    }
                }

                q = qmi(q, mod-2);
                last = (a[x] * q) % mod;

                for(int i = x-1;i >= 1;i --) {
                    if(a[i] == last) {
                        num ++;
                        last = (last * q) % mod;
                    }
                }
                ans = max(ans, num);
            }
        }
        if(ans >= (n+1)/2) printf("%d\n", ans);
        else printf("%d\n", -1);
    }
    return 0;
}