Problem

Given a string s of lowercase English letters, replace every character with the next letter in the alphabet: a → b, b → c, …, y → z, and z wraps around to a. Return the resulting string. Non-letter characters, if present, stay as they are.

Input / Output

• Input: string s.
• Output: the shifted string.

Examples

Example 1 Input: s = "abc" Output: "bcd"

Example 2 Input: s = "xyz" Output: "yza"

Example 3 Input: s = "hello world" Output: "ifmmp xpsme"

Constraints

• 1 <= s.length <= 10^5

Approach

For each lowercase letter c, compute (c - 'a' + 1) % 26 + 'a'. The modulo handles the wrap from z to a without a special case.

result = []
for c in s:
    if 'a' <= c <= 'z':
        result.append(chr((ord(c) - ord('a') + 1) % 26 + ord('a')))
    else:
        result.append(c)
return ''.join(result)

Work on a character buffer for linear time in languages with immutable strings.

Generalisation. This is a Caesar cipher with shift 1. Replace + 1 with + k for an arbitrary shift, and use (c - 'a' - k) % 26 (normalising negatives) to decode. Uppercase letters use the same formula with 'A'.

Complexity

• Time: O(n)
• Space: O(n) for the output.

Follow-ups

• Shift each character by its position in the string (i-th character shifts by i).
• Apply the shift only to vowels, or only to characters inside a given index range.
• Given the shifted string and the original, recover k.

Reference solution (Python)

def shift_next(s):
    out = []
    for c in s:
        if 'a' <= c <= 'z':
            out.append(chr((ord(c) - 97 + 1) % 26 + 97))
        else:
            out.append(c)
    return ''.join(out)

Test cases

Input	Expected
"abc"	"bcd"
"xyz"	"yza"
"hello world"	"ifmmp xpsme"
"z"	"a"