Reverse an integer

Hello World!

Today I'd like to share you how I solved reversing integer using JavaScript. Let me dive into the problem.

Input Format:

The input would be an integer 'x'

Constraints:

Should return the reverse of 'x'.
The reversed integer should be in the range [-2^31, 2^31 - 1]. Else, zero should be returned.

Solution:

The strategy I used to reverse the number is split and join. Which means, I will split each digit of a number and will store it in an array. Then I will reverse the array. After that I will join those array which would be the reversal of the number.

That is if the number is 123, then 123 -> ['1', '2', '3'] -> ['3', '2', '1'] -> 321

Okay, let me write the code for this

var reverse = function(x) {
    const reversedNumber = x.toString().split('').reverse().join('');
    return reversedNumber;
}

What I did in the code is I convert 'x' to a string, then split each digit and store in array using the split() function, then I reverse the Array using reverse() function, then I merge those reversed array using the join() function.

Now when we pass a number, we will get exact reverse of the number.

But the problem with the above code is for negative integers, it won't return reversed number. For example, if 'x' is -123, the above code will work like -123 -> ['-', '1', '2', '3'] -> ['3', '2', '1', '-'] -> 321-

In our case, it should be -321. How can we achieve that? For this, I've used a flag variable. Before reversing the number, I've added a check to find whether the number is positive or negative. If the number is negative the negativeFlag would be set true. Then I will reverse the absolute value of x (which means without considering the + or - signs) and finally if the negative flag is set to true, then we will negate the number. For changing the number to negative value, I have used the bitwise not operator (~). ~x will give one number less than the negative value of x. Which means if x is 5, then ~x would be -6. So I've added 1 to ~x. Below is the code implementation.

var reverse = function(x) {
    const negativeFlag = x<0;
    const reversedNumber = Math.abs(x).toString().split('').reverse().join('');
    return negativeFlag?~reversedNumber+1:reversedNumber;
};

Now we are done with reversal. Now if we look at the constraints, it is said that the reversed number should be in the range [-2^31, 2^31 - 1]. Let's implement this. For this I've used limitFlag variable, which would be set true if the reversed number is not in specified range. So, here is the final version of code.

var reverse = function(x) {
    const negativeFlag = x<0;
    const reversedNumber = Math.abs(x).toString().split('').reverse().join('');
    const limitFlag = reversedNumber<=Math.pow(-2, 31) || reversedNumber>Math.pow(2, 31);
    return limitFlag?0:negativeFlag?~reversedNumber+1:reversedNumber;
};

This problem is taken from Leet code. If you want to give a try with the problem, you can visit the above link.

Thank you!

Reverse an integer - JS

Input Format:

Constraints:

Solution: