Reverse Integer (Solution For Leetcode Problem #7)

  • Post last modified:July 16, 2022
  • Reading time:3 mins read

Understanding Problem

We have been given a signed 32-bit integer & we need to return the reverse of it.

if input_x = 345 return output_x = 543
if input_x = -345 return output_x = -543

My Thought About Solution

  • At first glance, the answer looks pretty straightforward. we just convert integer to string, then read each character and append it to the output string.
  • But we quickly realize that if we do so we also have to handle signs.
    That’s easy we can write code to take the first character as a substring from input and if it is a sign we can prepend to the reversed string.
  • Of course, we have to parse the to and from between integer and string
input_x = 345 
output_x = 543input_x = -345
output_x = 543- // oops we need to handle sign as well .
  • So, looks like we have a solution. Easy code!
  • But I forgot that reversed input can also fall outside of the range [-2³¹,2³¹-1], that case we need to handle.
  • if reverse integer falls out of range [-2³¹, 2³¹-1], then we need a data type that can store bigger integer than 32 bit. I think a long data type can store that integer since it’s 64 bit long.
  • So while parsing from reversed input string to long we will not get an exception for the reverse integer which falls out of the range [-2³¹, 2³¹-1].

Data structure: We are not using any collection, just primitive types integer, long, string, etc.
Time-complexity: O(length(N)) where N is the input integer
Space-complexity: O(1) constant space


1: Get the sign of the input integer (if it is - integer) . We will append it to our result integer.
2: convert integer input to long input 
3: get the absolute value of long input ( avoiding - sign)
4: get the string value of long input
5: reverse the long input and get the reversed string input
6: convert reversed string input to long input 
7: check if the long input is greater than max val of 32-bit integer
   if yes then return 0 else return integer value with sign 




  • Our submission beats just 7% of java submissions. Not that impressive. So I need to modify the solution.

Solution 2

  • In solution, I believe a lot of time is spent in parsing to and from between long, string, and integer.
  • So in our modified solution, this has to be minimum.
  • Can we achieve our result without converting the integer to string? can we just use integer and reverse it. Turns out that we can.
  • We can just use the same logic that we used on a string on our integer input which saves our runtime by avoiding integer to long to string etc..


Our result is very impressive this time. Runtime beats 100% of the submission. Nice job!


This Leetcode is good problem to understand

  • Typecasting In Java,
  • Max, and min value handling of integer
  • How to iterate over each digit in integer and play around with it.

Let me know if you have some opinion on the solution in the comment!

Happy Leetcoding!

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