TopCoder算法竞赛题3：SRM 147 DIV 2, 250-point

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.

Definition
Class: CCipher
Method: decode
Parameters: string, int
Returns: string
Method signature: string decode(string cipherText, int shift)
(be sure your method is public)

Constraints
cipherText has between 0 to 50 characters inclusive
each character of cipherText is an uppercase letter 'A'-'Z'
shift is between 0 and 25 inclusive

Examples
0)
"VQREQFGT"
2
Returns: "TOPCODER"

1)
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
10
Returns: "QRSTUVWXYZABCDEFGHIJKLMNOP"

2)
"TOPCODER"
0
Returns: "TOPCODER"

3)
"ZWBGLZ"
25
Returns: "AXCHMA"

4)
"DBNPCBQ"
1
Returns: "CAMOBAP"

5)
"LIPPSASVPH"
4
Returns: "HELLOWORLD"

Source code
#include <iostream>
#include <iterator>
using namespace std;

class CCipher
{
public:
string decode(string cipherText, int shift)
{
string s = "";
for(int i=0; i<(int)cipherText.size(); i++)
{
char c = cipherText[i];
c = ((c-'A')-shift+26)%26 + 'A';
s += c;
}
return s;
}
};

int main()
{
ostream_iterator<char> output(cout, "");
CCipher cipher;
string str = "VQREQFGT";
string Str = cipher.decode(str,2);
copy(Str.begin(), Str.end(), output);
cout<<endl;
return 0;
}