Reversing an integer mathematically in TypeScript

Reversing an integer is one of the most common problems for interviews. It can be easily done if we treat the number as a string, but there is a mathematical way of doing it.

The Idea

The basic idea involves extracting digits one by one from the number’s end, then building the reversed version of the number by shifting its digits left (multiplying by 10) and adding the extracted digit.

Steps

1. Initialize a variable to hold the reversed number, starting at 0.

2. Use a loop to extract digits from the original number until it becomes 0.

3. Extract the last digit using the modulus operator (% 10).

4. Append the digit to the reversed number by multiplying the current reversed number by 10 and adding the extracted digit.

5. Remove the last digit from the original number using integer division (Math.floor(number / 10)).

function numberReverser(num: number): number {
  let reversed = 0;
  let isNegative = num < 0;

  // Make the number positive if it's negative
  num = Math.abs(num);

  while (num > 0) {
    // Extract the last digit
    const lastDigit = num % 10;

    // Append it to the reversed number
    reversed = reversed * 10 + lastDigit;

    // Remove the last digit from the original number
    num = Math.floor(num / 10);
  }

  // Restore the sign if the number was negative
  return isNegative ? -reversed : reversed;
}

// Example Usage
console.log(numberReverser(123));  // Output: 321
console.log(numberReverser(-456)); // Output: -654
console.log(numberReverser(1000)); // Output: 1

Edge Cases

Negative Numbers: Properly handled by restoring the sign after reversing.

Numbers with Trailing Zeros: For example, 1000 becomes 1 since leading zeros are dropped in numeric representation.

Single-Digit Numbers: Remain unchanged.