Problem Statement

Given a character or string in lowercase, convert it to its uppercase equivalent.

Constraints:

• Input is a lowercase ASCII character or string.
• 1 ≤ length ≤ 10^5

Examples

Example 1:

Input: 'h'
Output: 'H'

Example 2:

Input: "hello123"
Output: "HELLO123"

Different Approaches

 1️⃣Using Standard Library Function (toupper())

Intuition:

C++ provides the toupper() function in <cctype> to convert a character to uppercase.

Code:

#include <iostream>
#include <cctype>
using namespace std;

string toUppercase(string str) {
    for (char &ch : str) {
        ch = toupper(ch);
    }
    return str;
}

int main() {
    string input = "hello123";
    cout << toUppercase(input);  // Output: HELLO123
}

Complexity Analysis:

• Time Complexity:O(n)
• Space Complexity:O(1)

2️⃣ Using ASCII Conversion

Intuition:

In ASCII:

• 'a' = 97, 'A' = 65 → difference = 32
• To convert: subtract 32 from lowercase letters

Code:

#include <iostream>
using namespace std;

string toUppercase(string str) {
    for (char &ch : str) {
        if (ch >= 'a' && ch <= 'z') {
            ch = ch - 32;
        }
    }
    return str;
}

int main() {
    string input = "world";
    cout << toUppercase(input);  // Output: WORLD
}

Complexity Analysis:

• Time Complexity:O(n)
• Space Complexity:O(1)

3️⃣ Using Bitwise Trick: ch & ‘_’

Intuition:

In ASCII:

• Bit 5 distinguishes lowercase from uppercase.
• AND-ing with underscore '_' = 0x5F clears bit 5

Why It Works:

Code:

#include <iostream>
using namespace std;

string toUppercase(string str) {
    for (char &ch : str) {
        if (ch >= 'a' && ch <= 'z') {
            ch = ch & '_';  // Bitwise trick
        }
    }
    return str;
}

int main() {
    string input = "cpp";
    cout << toUppercase(input);  // Output: CPP
}

Complexity Analysis:

• Time Complexity:O(n)
• Space Complexity:O(1)