UVALive 7457 Discrete Logarithm Problem

题目链接：https://icpcarchive.ecs.baylor.edu/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=5479

题意：给出a,b,p。求( a ^ x ) % p = b中的x。

思路：由于p不超过1W，那么%p后的答案也是小于p的，所以我们乘1W次如果能找到符合条件的x，那么就是存在的；否则无解。

#include <cstdio>
#include <cmath>
#include <cstring>
#include <string>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#include <stack>
#include <map>
#include <set>
#include <vector>
#include <sstream>
#include <queue>
#include <utility>
using namespace std;

#define rep(i,j,k) for (int i=j;i<=k;i++)
#define Rrep(i,j,k) for (int i=j;i>=k;i--)

#define Clean(x,y) memset(x,y,sizeof(x))
#define LL long long
#define ULL unsigned long long
#define inf 0x7fffffff
#define mod 100000007
const int maxn = 10009;

int p;
int a,b;

int main()
{
    cin>>p;
    while( scanf("%d%d",&a,&b) == 2 )
    {
        if ( b == 1 )
        {
            puts("0");
            continue;
        }
        int ans = 0;
        int temp = 1;
        bool f = false;
        rep(i,1,maxn)
        {
            ans++;
            temp = ( temp * a ) % p;
            if ( temp == b )
            {
                f = true;
                printf("%d\n",ans);
                break;
            }
        }
        if ( !f ) puts("0");
    }
    return 0;
}