LeetCode-in-All

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:

Here are the steps to solve the “Reverse Integer” problem:

Approach:

  1. Handle Special Case for 0:
    • If the given integer x is 0, return 0.
  2. Determine Sign:
    • Determine the sign of the integer x (positive or negative).
  3. Handle Negative Case:
    • If x is negative, update the sign and make x positive for the reversal process.
  4. Reverse the Digits:
    • Convert the positive x to a string.
    • Reverse the string.
  5. Handle Leading Zeros:
    • Remove leading zeros from the reversed string.
  6. Convert Back to Integer:
    • Convert the reversed string back to an integer.
  7. Apply Sign:
    • If the original x was negative, apply the negative sign to the reversed integer.
  8. Check for Overflow:
    • Check if the reversed integer is within the 32-bit integer range [-231, 231 - 1].
    • If not, return 0.
  9. Return Result:
    • Return the reversed integer.

Python Code:

class Solution:
    def reverse(self, x: int) -> int:
        # Handle special case for 0
        if x == 0:
            return 0

        # Determine sign
        sign = 1 if x > 0 else -1

        # Make x positive for the reversal process
        x = abs(x)

        # Reverse the digits
        reversed_str = str(x)[::-1]

        # Remove leading zeros
        reversed_str = reversed_str.lstrip('0')

        # Convert back to integer
        reversed_int = int(reversed_str)

        # Apply sign
        reversed_int *= sign

        # Check for overflow
        if reversed_int < -2**31 or reversed_int > 2**31 - 1:
            return 0

        # Return the reversed integer
        return reversed_int

# Example Usage:
solution = Solution()

# Example 1:
x1 = 123
print(solution.reverse(x1))  # Output: 321

# Example 2:
x2 = -123
print(solution.reverse(x2))  # Output: -321

# Example 3:
x3 = 120
print(solution.reverse(x3))  # Output: 21

# Example 4:
x4 = 0
print(solution.reverse(x4))  # Output: 0

This code defines a Solution class with a method reverse that takes an integer x as input and returns the reversed integer. The example usage demonstrates how to create an instance of the Solution class and call the reverse method with different inputs.

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