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string ceaser (string plain_text, int mode, int STEP)
{
    if (STEP >= 26) STEP %= 26;
    string cipher = "";
    for (int i = 0; i < plain_text.size(); i++) {
        if (plain_text[i] >= 48 && plain_text[i] <= 57) {
            cipher.push_back (plain_text[i]);
        } else if (plain_text[i] >= 65 && plain_text[i] <= 90) {
            cipher.push_back ( (char) (65 + (plain_text[i] + mode * STEP - 65) % 26));
        } else {
            cipher.push_back ( (char) (97 + (plain_text[i] + mode * STEP - 97) % 26));
        }
    }
    return cipher;
}
//这段代码在2022/6/10使用Win10和TDM-GCC 4.9.2 64-bit Release编译通过。

#include<iostream>
#include<string>
using namespace std;
enum MODE
{
    ENCODE = 1,
    DECODE = -1
};
string ceaser (string plain_text, int mode, int STEP)
//plain_text:要解密/加密的字符串
//mode      :区分解密/加密模式
//STEP      :偏移量
{
    if (STEP >= 26) STEP %= 26;
    string cipher = "";
    for (int i = 0; i < plain_text.size(); i++) {
        if (plain_text[i] >= 48 && plain_text[i] <= 57) {
            cipher.push_back (plain_text[i]);
        } else if (plain_text[i] >= 65 && plain_text[i] <= 90) {
            cipher.push_back ( (char) (65 + (plain_text[i] + mode * STEP - 65) % 26));
            //(1)
        } else {
            cipher.push_back ( (char) (97 + (plain_text[i] + mode * STEP - 97) % 26));
            //(2)
        }
    }
    return cipher;
}
string ceaser_encoding (string plain, int STEP)
//加密
{
    return ceaser (plain, ENCODE, STEP);
}
string ceaser_decoding (string plain, int STEP)
//解密
{
    return ceaser (plain, DECODE, STEP);
}
int main()
{
    string cipher;
    int n=1;
    //getline (cin, cipher);
    cipher="helloworld";
    string res = ceaser_encoding (cipher, n);
    cout << "密文：" << res << endl;
    cout << "明文：" << ceaser_decoding (res, n) << endl;
    return 0;
}