Caesar Cipher

Solution

Iterative - O(n) time - O(1) space

Notes:

1. We do k = k % 26 first because any conversions over the number of alphas would just turn things around needlessly so this short-circuits the if conditions if k > 26
2. We use (int)'A' and others instead of ascii characters for readability, and because nobody remembers ascii codes.
3. We add 1 to first line because we want inclusive count and not just difference, which doesn’t count ‘A’ in the range
4. We remove 1 from the ascii modding because we actually want to add whatever the value prior to A or a is so that we can include the first letter of the alphabet

string caesarCipher(string s, int k) {
	k = k % ((int)'Z' - (int)'A' + 1);
	for (int i = 0; i < s.length(); i++) {
		if (!isalpha(s[i])) continue;
		int ascii = (int)s[i] + k;
		if (isupper(s[i]) && ascii > (int)'Z') 
			ascii = ascii % (int)'Z' + (int)'A' - 1;
		if (islower(s[i]) && ascii > (int)'z') 
			ascii = ascii % (int)'z' + (int)'a' - 1;
		s[i] = (char)ascii;
	}
	return s;
}