CUBE-T009: Z-Encryption

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

int main(){

    int key, len;
    scanf("%d", &key);
    string str;
    cin >> str;
    if(key == 1){
        cout << str;
        return 0;
    }
    len = str.size();
    string ans(len + 1, '\0');

    // Up Zip
    int jump = 3 * key - 4;
    int sIdx = 0;
    int aIdx = key - 1;
    while(aIdx < len){
        for(int i = 0; i < key && aIdx < len; ++i){
            ans[aIdx++] = str[sIdx++];
        }
        aIdx += jump;
    }

    // Middle Zip
    for(int l = 0; l < key - 2; ++l){
        int up = key + l * 2;
        int down = key + (key - 3 - l) * 2;
        bool isUp = true;
        aIdx = key - 2 - l;
        while(aIdx < len){
            ans[aIdx++] = str[sIdx++];
            if(isUp){
                aIdx += up;
            } else {
                aIdx += down;
            }
            isUp = !isUp;
        }
    }

    // Down Zip
    ans[0] = str[sIdx++];
    aIdx = jump + 1;
    while(aIdx < len){
        for(int i = 0; i < key && aIdx < len; ++i){
            ans[aIdx++] = str[sIdx++];
        }
        aIdx += jump;
    }
    printf("%s", ans.c_str());

    return 0;
}