LeetCode 504: Base 7

Meta Data

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Difficulty: easy First Attempt: 2022-08-25

• Submissions: Runtime Error → Accepted

Approach

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Base conversion (Math Template 7): repeatedly take % 7 of abs(num) for the lowest digit and // 7 to move on, reverse, and prepend - for negatives. With num = 0 the loop never runs, so return "0" directly.

My Attempts

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The first submission pasted the wrong problem’s code, a BST-mode solution (AttributeError). Seconds later the right code passed.

Solution

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class Solution:
    def convertToBase7(self, num: int) -> str:
        if num == 0:
            return "0"
        n, digits = abs(num), []
        while n:
            digits.append(str(n % 7))
            n //= 7
        return ("-" if num < 0 else "") + "".join(reversed(digits))

Complexity

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Time O(log n), space O(log n).

Mistakes to Avoid

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• Handle 0 separately.
• Work on the absolute value; Python’s % gives a non-negative remainder for negatives.

Interview Explanation

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I take remainders mod 7 of the absolute value for each digit, divide, reverse at the end, and add the sign; 0 returns directly. Time O(log n).