TopCoder SRM 147, DIV II, CCipher

Problem Statement

Julius Caesar used a system of cryptography, now known as Caesar Cipher, which shifted each letter 2 places further through the alphabet (e.g. 'A' shifts to 'C', 'R' shifts to 'T', etc.). At the end of the alphabet we wrap around, that is 'Y' shifts to 'A'.

We can, of course, try shifting by any number. Given an encoded text and a number of places to shift, decode it.

For example, "TOPCODER" shifted by 2 places will be encoded as "VQREQFGT". In other words, if given (quotes for clarity) "VQREQFGT" and 2 as input, you will return "TOPCODER". See example 0 below.

#include <iostream>
#include <string>
#include <vector>
#include <deque>
#include <set>
#include <map>
#include <list>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <bitset>
#include <algorithm>
#include <sstream>
#include <cstring>
using namespace std;
typedef long long int lli;
typedef long unsigned long llu;

//definitions
#define mem_prep(varx) memset(varx,0,sizeof(varx))
#define all(x)  (x).begin(),(x).end()
#define pb push_back
#define mp make_pair
#define sz(v) ((int)(v).size())
#define rep(i,a,n) for(int i=(a);i<(n);i++)
#define repe(i,a,n)  for(int i=(a);i<=(n);i++)
#define forsz(i,a,v) rep(i,a,sz(v))
#define repsz(i,v) rep(i,0,sz(v))
#define vi vector<int>
#define vs vector<string>

class CCipher {

public:

string decode(string s, int shift) {
 repsz(i, s){
   s[i]-=shift;
   if(s[i]<'A')s[i]+=26;
 }
 return s;
}

};