Decrypt

input/code.cpp: In function 'char applyF(char, int)':
input/code.cpp:30:14: warning: array subscript has type 'char' [-Wchar-subscripts]
         c=F[c];
              ^
input/code.cpp: In function 'int main()':
input/code.cpp:41:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0; i<f.size(); i++)
                           ^
input/code.cpp:61:30: warning: array subscript has type 'char' [-Wchar-subscripts]
                 char d=F[iter];
                              ^
input/code.cpp:64:26: warning: array subscript has type 'char' [-Wchar-subscripts]
                     d=F[d];
                          ^
input/code.cpp:82:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0; i<s.size(); i++)
                           ^
input/code.cpp:83:20: warning: array subscript has type 'char' [-Wchar-subscripts]
         if(cnt[s[i]]!=1){
                    ^
input/code.cpp:88:27: warn...

#include <iostream>
#include <vector>
#include <stack>
#include <queue>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <unordered_set>
#include <stdio.h>
#include <string.h>
#include <unordered_map>
#include <fstream>
#include <set>
#include <map>

#define MOD 1000000007
#define ll long long
#define N 20000
#define float double

using namespace std;

int F[256];
int G[256];
int cnt[256];
int Ginv[256];

char applyF(char c, int n){
    for(int i=0; i<n; i++)
        c=F[c];
    return c;
}

int main(){
    string f;
    int k;
    cin>>f>>k;
    string s;
    cin>>s;

    for(int i=0; i<f.size(); i++)
        F['a' + i] = f[i];

    for(int i='a'; i<='z'; i++)
        G[i]=i;

    if(k<100){
        for(int i='a'; i<='z'; i++)
            G[i]=applyF(i, k);
    }else{
        for(int i='a'; i<='z'; i++){
            int syklialkaa=-1;
            int syklinpituus=-1;

            // VITUN PURKKAFEST ":D"

            for(int alq=0; alq<30; alq++){
                char iter=applyF(i, alq);
                int maar=1;

                char d=F[iter];
                while(maar<30 && d!=iter){
                    maar++;
                    d=F[d];
                }

                if(d==iter){
                    syklialkaa=alq;
                    syklinpituus=maar;
                    break;
                }
            }

            G[i]=applyF(i, syklialkaa +((k-syklialkaa)%syklinpituus));
        }
    }

    for(int i='a'; i<='z'; i++)
        cnt[G[i]]++;


    for(int i=0; i<s.size(); i++)
        if(cnt[s[i]]!=1){
            cout<<"impossible"<<endl;
            return 0;
        }

    for(int i=0; i<s.size(); i++){
        for(int c='a'; c<='z'; c++){
            if(G[c]==s[i]){
                s[i]=c;
                break;
            }
        }

    }
    cout<<s<<endl;
    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED

Test 24

Verdict: ACCEPTED

Test 25

Verdict: ACCEPTED

Test 26

Verdict: ACCEPTED

Test 27

Verdict: ACCEPTED

Test 28

Verdict: ACCEPTED

Test 29

Verdict: ACCEPTED

Test 30

Verdict: ACCEPTED