LeetCode-in-Java

7. Reverse Integer

Medium

Given a signed 32-bit integer x, return x with its digits reversed. If reversing x causes the value to go outside the signed 32-bit integer range [-231, 231 - 1], then return 0.

Assume the environment does not allow you to store 64-bit integers (signed or unsigned).

Example 1:

Input: x = 123

Output: 321

Example 2:

Input: x = -123

Output: -321

Example 3:

Input: x = 120

Output: 21

Example 4:

Input: x = 0

Output: 0

Constraints:

• -231 <= x <= 231 - 1

To solve the Reverse Integer problem in Java using a Solution class, we’ll follow these steps:

1. Define a Solution class with a method named reverse.
2. Initialize variables to keep track of the reversed integer (rev), the sign of the input integer (sign), and the absolute value of the input integer (x).
3. Iterate through each digit of the input integer x:
   • Extract the least significant digit using the modulo operator.
   • Update the reversed integer rev by multiplying it by 10 and adding the extracted digit.
   • Update x by removing the least significant digit using integer division.
4. Check if the reversed integer rev overflows the signed 32-bit integer range. If so, return 0.
5. Return the reversed integer rev with the appropriate sign.

Here’s the implementation:

public class Solution {

    public int reverse(int x) {
        int rev = 0;
        int sign = (x < 0) ? -1 : 1;
        int limit = Integer.MAX_VALUE / 10;
        int absX = Math.abs(x);

        while (absX > 0) {
            int digit = absX % 10;
            absX /= 10;

            if (rev > limit || (rev == limit && digit > 7)) {
                return 0;
            }

            if (rev < -limit || (rev == -limit && digit < -8)) {
                return 0;
            }

            rev = rev * 10 + digit;
        }

        return rev * sign;
    }

    public static void main(String[] args) {
        Solution solution = new Solution();

        // Test cases
        int x1 = 123;
        System.out.println("Example 1 Output: " + solution.reverse(x1));

        int x2 = -123;
        System.out.println("Example 2 Output: " + solution.reverse(x2));

        int x3 = 120;
        System.out.println("Example 3 Output: " + solution.reverse(x3));

        int x4 = 0;
        System.out.println("Example 4 Output: " + solution.reverse(x4));
    }
}

This implementation provides a solution to the Reverse Integer problem in Java.

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