Reverse Integer (Solution For Leetcode Problem #7)

Java Solution for https://leetcode.com/problems/reverse-integer/

Originally published at https://asyncq.com

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 = 543
input_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

Pseudocode

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…