If you don't feel like reviewing my code at least give me some tests please.
So... um... I think my code should work... I tested it along with an accepted program... and it worked fine... Here's the code:

#define DM 1001
#include <bitset>
#include <iostream>
#include <set>
using namespace std;

bitset <DM> bs;
bool verif;
int n, c, lg[DM], mp[DM];
long long v;
set <int> s;

void recursiv (int a)
{
    bs[a] = 1;
    ++v;
    if (s.find(mp[a]) != s.end())
        return;
    s.insert(a);
    recursiv(mp[a]);
}

int main ()
{
    cin >> n;
    for (int i = 1; i <= n; ++i)
    {
        cin >> mp[i];
        if (mp[i] == i)
            bs[i] = 1;
    }
    for (int i = 1; i <= n; ++i)
        if (!bs[i])
        {
            recursiv(i);
            s.clear();
            lg[++c] = v;
            v = 0;
        }
    for (v = lg[1]; !verif; ++v)
    {
        verif = 1;
        for (int i = 1; i <= c; ++i)
            if (v%lg[i] != 0)
                verif = 0, i = c + 1;
    }
    cout << v - 1;
    return 0;
}

Your solution is good, but try to fix these problems :

Test
3
1 2 3

and

don't use set. You will get time limit for n = 1000

sorry for my poor english!