傳送門：http://www.lydsy.com/JudgeOnline/problem.php?id=3613

【題解】

其實挺顯然。。就是最大逆序對差/2

# include <stdio.h>
# include <string.h>
# include <iostream>
# include <algorithm>
// # include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef unsigned 
 
long long ull;
const int M = 5e6 + 10;

# define RG register
# define ST static

int n, SA, SB, SC, SD;
int a[M], mod; 

inline int F(int x) {
    ll ret = 1ll * SA * x % mod * x % mod * x % mod + 1ll * SB * x % mod * x % mod + 1ll * SC * x % mod + SD;
    return int(ret % mod);
}

int main() {
    cin 
 
>> n >> SA >> SB >> SC >> SD >> a[1] >> mod;
    for (int i=2; i<=n; ++i) { 
        a[i] = F(a[i-1]) + F(a[i-2]); 
        if(a[i] >= mod) a[i] -= mod;
    }
    int mx = -1, ans = 0;
    for (int i=1; i<=n; ++i) {
        if(mx >= a[i]) ans = max(ans, (mx-a[i]+1
 
)>>1);
        mx = max(mx, a[i]);
    }
    cout << ans << endl;
    return 0;
}

bzoj3613 [Heoi2014]南園滿地堆輕絮