Problem

Given a string s, return a new string in which every uppercase letter is converted to lowercase and every lowercase letter is converted to uppercase. Non-alphabetic characters (digits, spaces, punctuation) remain unchanged.

Input / Output

• Input: string s.
• Output: the case-toggled string.

Examples

Example 1 Input: s = "Hello World" Output: "hELLO wORLD"

Example 2 Input: s = "aBc123!" Output: "AbC123!"

Example 3 Input: s = "" Output: ""

Constraints

• 0 <= s.length <= 10^5
• ASCII characters only (extend with Unicode-aware library calls if required).

Approach

Iterate over each character and branch on its range:

• 'a'..'z' → subtract 32 (or c - 'a' + 'A').
• 'A'..'Z' → add 32.
• otherwise → copy unchanged.

Bit trick. In ASCII, uppercase and lowercase letters differ only in bit 5 (value 32). So for any letter, c ^ 32 flips its case in one operation, and the check isLetter(c) guards non-letters. Interviewers often ask you to explain why 'A' = 65 = 0b1000001 and 'a' = 97 = 0b1100001.

Build the result in a character array or string builder rather than concatenating strings in a loop (which is O(n²) in languages with immutable strings).

Complexity

• Time: O(n)
• Space: O(n) for the output; O(1) extra if done in place on a mutable buffer.

Follow-ups

• Toggle case only for alternate characters or only within words.
• Handle non-ASCII scripts using library functions such as swapcase() in Python.
• Count how many characters changed.

Reference solution (Python)

def toggle_case(s):
    out = []
    for c in s:
        if 'a' <= c <= 'z' or 'A' <= c <= 'Z':
            out.append(chr(ord(c) ^ 32))
        else:
            out.append(c)
    return ''.join(out)

Test cases

Input	Expected
"Hello World"	"hELLO wORLD"
"aBc123!"	"AbC123!"
""	""
"ZZZ"	"zzz"