Problem could be found on Leetcode Here

Problem Description

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: [−2³¹, 2^{31 − 1}]. For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.

My Solution

Code

public class Solution {
    public int Reverse(int x) {
        if(x == 0 || x == int.MinValue) return 0;
        int ans = 0;
        int pre_overflow = int.MaxValue / 10;
        int suf_overflow = int.MaxValue % 10;
        bool neg_tag = false;
        if(x < 0){
            neg_tag = true;
        }
        // return directly if x == 0
        x = Math.Abs(x);
        while(x > 0){
            int res = x % 10;
            // if the current answer > int.MaxValue % 10, in the following step, ans = ans * 10, it will cause overflow
            if(ans > pre_overflow) return 0;
            // if answer == int.MaxValue // 10, check the comming digit.
            if(ans == pre_overflow && res > suf_overflow) return 0;
            ans = ans * 10 + res;
            x = x / 10;
        }
        if(neg_tag == true){
            return -ans;
        }
        else return ans;
    }
}

Explain

Time Complexity: O(n) - n is the length of (string)x

This problem have multiple solutions.

String

Convert the input int into string, check if it has ‘-‘ at the beginning of the string or not. If yes, set variable neg_tag as true and remove str[0].

Then reverse the string by reversed_s = new string(s.Reverse().ToCharArray()), if neg_tag is true, do reversed_s.Insert(0, "-")

At the end convert string to int and return

Long

Set variable ans as long type, let x % 10 become current digit, return 0 if x < −2³¹ or x > 2^{31 − 1}, else return x

Remenber you cannot set ans as int type without check, otherwise it might cause error if ans > 2^{31 − 1}

Try/catch

Quite similar with “Long” solution but set ans as int type, if error(overflow), return -1 in catch.

Int Only Solution

It is what my code does. Explained in the comment.

Leetcode 7. Reverse Integer Csharp(C#) Solution

Leetcode With Csharp series tutorial