caesar.js

/*
*
 * 凯撒密码 - 简单的替换加密
 *
 * 问题：将字母表中的每个字母移动固定位数
 *
 * 核心思想：
 * - 字母替换
 * - 循环移位
 * - 保持大小写
 *
 * 时间复杂度: O(n)
 * 空间复杂度: O(n)
*/

function caesarEncrypt(text, shift) {
    let result = "";
    for (let i = 0; i < text.length; i++) {
        let c = text[i];
        if (c >= 'A' && c <= 'Z') {
            result += String.fromCharCode((c.charCodeAt(0) - 65 + shift) % 26 + 65);
        } else if (c >= 'a' && c <= 'z') {
            result += String.fromCharCode((c.charCodeAt(0) - 97 + shift) % 26 + 97);
        } else {
            result += c;
        }
    }
    return result;
}

function caesarDecrypt(text, shift) {
    return caesarEncrypt(text, 26 - (shift % 26));
}

function main() {
    console.log("=== 凯撒密码 ===");
    const text = "Hello, World!";
    const shift = 3;
    console.log("明文: " + text);
    console.log("移位数: " + shift);
    const encrypted = caesarEncrypt(text, shift);
    console.log("加密后: " + encrypted);
    const decrypted = caesarDecrypt(encrypted, shift);
    console.log("解密后: " + decrypted);
    console.log("验证: " + (text === decrypted));
}

main();