当你发现要优化一下时间复杂度就可以暴力过be like

#include <bits/stdc++.h>
#define int long long
using namespace std;
#define endl '\n'
#define pb push_back
#define ull unsigned long long
#define all(a) a.begin(), a.end()
#define vi vector<int>
#define vii vector<vector<int>>
#define fi first 
#define se second
#define vs vector<string>
#define eb emplace_back
#define in insert
#define pf push_front
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep1(i, n) for (int i = 1; i <= (n); i++)
const int inf = 2e18 + 9 ;
const int mod1 = 1e9 + 7 ; 
const int mod2 = 998244353 ; 
typedef pair<int, int> pll;
typedef long double db;

inline int gcd(int a , int b) {return b ? gcd(b , a % b) : a;};
template<class T>inline void read(T &res){char c;T flag=1;while((c=getchar())<'0'||c>'9')if(c=='-')flag=-1;res=c-'0';while((c=getchar())>='0'&&c<='9')res=res*10+c-'0';res*=flag;}
inline int qsm(int a , int b){int res = 1 ; while(b){if(b & 1){res *= a ; }b >>= 1 ; a *= a ; }return res ; }
int dir[4][2] = {{1 , 0} , {-1 , 0} , {0 , 1} , {0 , -1}};
int dirx[8] = {-1, -1, -1, 0, 0, 1, 1, 1}; 
int diry[8] = {-1, 0, 1, -1, 1, -1, 0, 1};
void work() 
{
    int n , a , m ; cin >> n >> a >> m ; 
    vi b(n + 1 , 0) ; 
    for(int i = 1 ; i <= n ; i++)
    {
        if(i != 1)
        {
            a = (a + 7 * i) % m ; 
        }
        int h = i ; 
        for(int j = 1 ; j <= n ; j++)
        {
            int num = h * j ; 
            if(num > n)
            {
                break ; 
            }
            b[num] += a ; 
        }
    }
    int ans = 0 ;
    for(int i = 1 ; i <= n ; i++)
    {
        ans ^= b[i] ; 
    }
    cout << ans << endl ; 
}
signed main() 
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int t = 1;
    while (t--) 
    {
        work();
    }
    return 0;
}