Coding problem · Easy · Basics

Caesar Cipher

Easy Algorithms & data structures

Encrypt s with a Caesar cipher: shift each letter forward by shift places in the alphabet, wrapping around (z + 1 = a). Keep upper and lower case, and leave other characters unchanged. 0 <= shift <= 25.

Example 1

Input: s = "abc", shift = 1
Output: "bcd"

Example 2

Input: s = "Hello, World!", shift = 3
Output: "Khoor, Zruog!"

Hints

Hint 1

Turn a letter into a number 0-25 by subtracting the code of 'a' (or 'A').

Hint 2

Add the shift, take the remainder modulo 26, and turn it back into a letter.

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Solution and explanation

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Time O(n) · Space O(n)

For each letter, compute (code - base + shift) % 26 + base, where base is the code of 'a' or 'A'. The modulo handles the wrap-around.

Reference solution

function caesarCipher(s, shift) {
  return s.replace(/[a-z]/gi, (ch) => {
    const base = ch <= 'Z' ? 65 : 97;
    return String.fromCharCode(((ch.charCodeAt(0) - base + shift) % 26) + base);
  });
}

function caesarCipher(s: string, shift: number): string {
  return s.replace(/[a-z]/gi, (ch) => {
    const base = ch <= 'Z' ? 65 : 97;
    return String.fromCharCode(((ch.charCodeAt(0) - base + shift) % 26) + base);
  });
}

def caesar_cipher(s: str, shift: int) -> str:
    out = []
    for ch in s:
        if ch.isalpha() and ch.isascii():
            base = ord('A') if ch.isupper() else ord('a')
            out.append(chr((ord(ch) - base + shift) % 26 + base))
        else:
            out.append(ch)
    return ''.join(out)

<?php

function caesarCipher(string $s, int $shift): string
{
    $out = '';
    foreach (str_split($s) as $ch) {
        if (ctype_alpha($ch)) {
            $base = ctype_upper($ch) ? 65 : 97;
            $out .= chr((ord($ch) - $base + $shift) % 26 + $base);
        } else {
            $out .= $ch;
        }
    }
    return $out;
}

class Solution {
    public String caesarCipher(String s, int shift) {
        StringBuilder sb = new StringBuilder();
        for (char ch : s.toCharArray()) {
            if (ch >= 'a' && ch <= 'z') sb.append((char) ((ch - 'a' + shift) % 26 + 'a'));
            else if (ch >= 'A' && ch <= 'Z') sb.append((char) ((ch - 'A' + shift) % 26 + 'A'));
            else sb.append(ch);
        }
        return sb.toString();
    }
}

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