LeetCode 7: Reverse Integer

Meta Data

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Difficulty: medium First Attempt: 2019-09-16

• Submissions: Wrong Answer → Accepted ×8

Approach

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Remember the sign, reverse the digits of abs(x) with % 10 and // 10 (Math Template 5), then restore the sign. Return 0 if the result leaves [-2^31, 2^31 - 1]. Python integers do not overflow, so checking at the end is fine; in C++ or Java check before each multiplication by 10.

My Attempts

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In 2019 the first version dropped the minus sign via str(x)[:0:-1] and never put it back: -29 returned 92. Restoring the sign passed.

Solution

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class Solution:
    def reverse(self, x: int) -> int:
        sign = -1 if x < 0 else 1
        x, rev = abs(x), 0
        while x:
            rev = rev * 10 + x % 10
            x //= 10
        rev *= sign
        return rev if -2**31 <= rev <= 2**31 - 1 else 0

Complexity

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Time O(log x), space O(1).

Mistakes to Avoid

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• Restore the minus sign after reversing a negative number.
• Check the range on the reversed result.

Interview Explanation

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I save the sign, reverse the absolute value digit by digit, restore the sign, and return 0 outside the signed 32-bit range. Python cannot overflow, so a final check suffices; other languages must check before each multiplication by 10.