LeetCode Problem 7. Reverse Integer

In this segment, I go through solving leetcode problems for my own practice. I walk through my thinking process so that we can all learn together.

July 9, 2020 - 3 minute read -
algorithms leetcode programming problem-solving

LeetCode Problem


Given a 32-bit signed integer, reverse digits of an integer.

Example 1:

Input: 123
Output: 321
Example 2:

Input: -123
Output: -321
Example 3:

Input: 120
Output: 21

Note: Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−2^31, 2^31 − 1]. For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.


var reverse = function(x) {
    var isNegative = false
    if(x < 0) {
       isNegative = true
    const str = Math.abs(x).toString()

    if(str.length === 1) {
       return x
    var revStr = ""
    const len = str.length
    for(var i = len - 1; i >= 0 ; i--) {
        revStr += str[i]
    const reverseNum = +revStr
    if (reverseNum > 0x7FFFFFFF) {
        return 0
    return isNegative ? -reverseNum : reverseNum


The input is a signed integer

Max 32 bit signed integer is But more efficient is using constant - 0x7FFFFFFF

Can it be converted to string and then converted back to integer

I bet this has some easy solution using bit manipulation But i hate bit manipulation

Let s go ahead with converting to string and back to integer We can maintain the sign in a variable

If the number is single digit we can just return it

Math.abs(x) is important because we are maintaining the sign separately

+revStr is a cool trick to convert string back to integer

str.reverse() can make this much more cleaner. But, I like to write my own logic for reverse. It’s better to do it this way in the interviews.


Runtime: 84 ms, faster than 75.52% of JavaScript online submissions for Reverse Integer.

Memory Usage: 37.7 MB, less than 14.63% of JavaScript online submissions for Reverse Integer.


This may not be the optimal solution. And that’s okay. The purpose here is to practice problem solving and have fun with algorithms. I am constantly learning new optimized solutions for these problems.

Please comment below if you have a better solution. Let’s learn algorithms and data structures together.

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