How to Reverse a Number Mathematically

Here’s a simple method for getting the reverse of a positive integer.

1. Get the last digit

let lastDigit = num % 10

2. Append the last digit to the reverse

reversedNum = (reversedNum * 10) + lastDigit

3. Remove the last digit from the original number

num = num / 10

Putting it all together into a loop, we have:

func reverse(_ input: Int) -> Int {
    var num = input // create a mutable copy
    var reversedNum = 0 // result

    while num > 0 {
        let lastDigit = num % 10
        reversedNum = (reversedNum * 10) + lastDigit
        num = num / 10
    }

    return reversedNum
}

reverse(123) // returns 321