十分不解！

#include<iostream>
#include<cstring>
#include<cstdio>
#define MAXN 10086
using namespace std;
char s[] = "zabcdefghijklmnopqrstuvwxy";
char sc[MAXN], c[MAXN];
int n;
int main()
{
    scanf("%d", &n);
    int k = 0;
    cin>>c;
    for(int i = 0;i <= strlen(c); i++)
      for(int j = 0;j <= 25; j++) {
          if(c[i] == s[j]) {
              //cout<<c[i]<<" "<<s[j];
              sc[k] = s[ (j + n)%26 ];
              k++;
          }
      }//cout<<endl;
      for(int i = 0;i <= k;i++)cout<<sc[i];
    /*for(int i = 0;i <= strlen(c); i++) cout<<c[i]<<" ";
    cout<<endl;
    for(int j = 0;j <= 25; j++)cout<<s[j]<<" ";*/
    return 0;
}