Coding

Change Case of Each Character in a String

Asked at

Amazon

1Give yourself 5 minutes

2Answer out loud, not in your head

3Then compare with the answer below

The problem

Given a string s, write a program to invert the case of each character:

Convert **lowercase **letters to uppercase
Convert **uppercase **letters to lowercase
Leave non-alphabetic characters unchanged

Input: s = "javA" **Output: **"JAVa" **Explanation: **j → J, a → A, v → V, A → a

Input: s = "take u forward IS Awesome" **Output: **"TAKE U FORWARD is aWESOME" Explanation: All alphabet cases inverted; spaces preserved

**Input: **s = ""

0 <= s.length <= 105 The string may contain:

lowercase & uppercase English letters
spaces, digits, and special characters

cpp

class Solution {
public:
    string toggleCase(const string& s) {
        // Your code goes here
    }
};

java

class Solution {
    public String toggleCase(String s) {
        // Your code goes here
    }
}

python

class Solution:
    def toggle_case(self, s: str) -> str:
        # Your code goes here

javascript

class Solution {
    toggleCase(s) {
        // Your code goes here
    }
}

csharp

class Solution
{
    public string ToggleCase(string s)
    {
        // Your code goes here
    }
}

func toggleCase(s string) string {

}

Stuck? Show a way to structure it+

01Inspect every character once.
02Invert only alphabetic case.
03Leave digits, punctuation, and whitespace unchanged.
04Build or mutate the output safely.

Reference answer

Iterate through the string. If a character is lowercase, convert it to uppercase; if uppercase, convert it to lowercase; otherwise copy it unchanged. Standard library character predicates are clearer than ASCII arithmetic unless the specification guarantees ASCII. The algorithm is O(n) time and O(n) space for immutable strings, or O(1) extra space if a mutable character array is allowed. Preserve spaces and punctuation exactly. In an interview, make the invariant explicit before writing code, trace a boundary example that breaks a tempting shortcut, state what each stored value represents, and explain why every update preserves correctness. This verification step also surfaces empty-input, duplicate-value, and numeric-limit cases.

Then expect these follow-ups

How do locale-sensitive case rules complicate this?
Tests: internationalization
How would you mutate a byte buffer in place?
Tests: representation

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