BSGS——大步小步算法

题目1：

Discrete Logging UVA - 10225

分析：

BSGS模板

讲解：

BSGS讲解

代码：

#include <iostream>
#include <map>
#include <cmath>
#include <algorithm>
using namespace std;
typedef long long ll;
//typedef __int128 lll;
#define print(i) cout << "debug: " << i << endl
#define close() ios::sync_with_stdio(0), cin.tie(0), cout.tie(0)
#define mem(a, b) memset(a, b, sizeof(a))
const ll mod = 1e9 + 7;
const int maxn = 2e6;
const int inf = 0x3f3f3f3f;

ll qpow(ll a, ll n, ll mo)
{
    ll res = 1;
    while(n)
    {
        if(n & 1) res = res * a % mo;
        a = a * a % mo;
        n >>= 1;
    }
    return res;
}

int main()
{
    ll p, b, n; 
    while(cin >> p >> b >> n)
    {
        map<ll, ll> vis;
        ll m = ceil(sqrt(p));
        for(int j = 0; j <= m; j++)
            vis[qpow(b, j, p) * n % p] = j;
        ll t = qpow(b, m, p);
        ll ans = 1;
        int flag = 0;
        for(int i = 1; i <= m; i++)
        {
            ans = ans * t % p;
            if(vis.count(ans))
            {
                ll res = i * m - vis[ans];
                cout << (res % p + p) % p << endl;
                flag = 1;
                break;
            }
        }
        if(!flag) cout << "no solution" << endl;
    }
}