Cryptopangrams (Test set 1)

#include <bits/stdc++.h>
using namespace std;

int main()
{
    // build sieve
    bool used[10010] = {};
    vector<int> primes;
    for (int i = 2; i <= 10000; ++i) {
        if (used[i]) continue;
        primes.push_back(i);
        for (int j = i+i; j <= 10000; j += i)
            used[j] = true;
    }

    int T;
    scanf("%d", &T);
    for (int t = 1; t <= T; ++t) {
        // input
        int n, l;
        scanf("%d%d", &n, &l);
        assert(n <= 10000);
        int val[l], P[l+1];
        for (int i = 0; i < l; ++i)
            scanf("%d", &val[i]);
        // find suitable starting point
        int st;
        for (int i = 0; i < l; ++i) {
            int sol = 0;
            for (auto p : primes) {
                if (val[i]%p == 0 && val[i+1]%p == 0)
                    sol = sol == 0 ? p : -1;
            }
            if (sol > 0) {
                st = i;
                P[i] = val[i]/sol;
                break;
            }
        }
        // go left and right
        for (int i = st+1; i < l+1; ++i)
            P[i] = val[i-1]/P[i-1];
        for (int i = st-1; i >= 0; --i)
            P[i] = val[i]/P[i+1];
        // create map to characters
        map<int, char> M;
        for (int i = 0; i < l+1; ++i)
            M[P[i]] = 0;
        char cnt = 'A';
        for (auto &m : M) // &m = allows editing directly in the map
            m.second = cnt++;
        printf("Case #%d: ", t);
        for (int i = 0; i < l+1; ++i)
            printf("%c", M[P[i]]);
        printf("\n");
    }

    return 0;
}