《POJ-2369》

求置换群的阶数：https://www.cnblogs.com/zwjzwj/p/14819849.html

注意的是ans相乘后可能爆int。

// Author: levil
#include<iostream>
#include<stdio.h>
#include<queue>
#include<algorithm>
#include<math.h>
#include<stack>
#include<map>
#include<limits.h>
#include<vector>
#include<string.h>
#include<string>
using namespace std;
using namespace std;
typedef long long LL;
typedef pair<LL,int> pii;
const int N = 1e3 + 5;
const int M = 1e5 + 5;
const LL Mod = 199999;
#define pi acos(-1)
#define INF 1e9
#define dbg(ax) cout << "now this num is " << ax << endl;
namespace FASTIO{
    inline LL read(){
        LL x = 0,f = 1;char c = getchar();
        while(c < '0' || c > '9'){if(c == '-') f = -1;c = getchar();}
        while(c >= '0' && c <= '9'){x = (x<<1)+(x<<3)+(c^48);c = getchar();}
        return x*f;
    }
    void print(int x){
        if(x < 0){x = -x;putchar('-');}
        if(x > 9) print(x/10);
        putchar(x%10+'0');
    }
}
using namespace FASTIO;

int n,a[N],p[N],pp[N],cnt[N],tot = 0;
bool solve() {
    for(int i = 1;i <= n;++i) {
        pp[i] = a[p[i]];
    }
    for(int i = 1;i <= n;++i) {
        if(pp[i] == i && cnt[i] == 0) cnt[i] = tot;
        p[i] = pp[i];
    }
    for(int i = 1;i <= n;++i) if(cnt[i] == 0) return false;
    return true;
}
int main() {
    while(cin >> n) {
        memset(cnt,0,sizeof(cnt));
        for(int i = 1;i <= n;++i) a[i] = read(),p[i] = a[i];
        LL ans = 1;
        tot = 1;
        for(int i = 1;i <= n;++i) if(p[i] == i) cnt[i] = 1;
        while(1) {
            ++tot;
            if(solve()) break;
        }
        for(int i = 1;i <= n;++i) if(cnt[i] != 0) ans = ans * cnt[i] / __gcd(ans,1LL * cnt[i]);
        printf("%lld\n",ans);
    }


    system("pause");
    return 0;
}