LeetCode 415: Add Strings ·

Table of Contents

Meta Data
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Difficulty: easy First Attempt: 2022-08-24

Submissions: Runtime Error ×4 → Wrong Answer ×2 → Accepted

Approach
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Column addition (Math Template 6): i and j walk from the ends of the two strings, treating a finished one as 0. Each step computes total = a + b + carry, writes total % 10 and carries total // 10. If a carry remains after the loop, append one more digit. Reverse at the end.

My Attempts
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This took 7 submissions:

Attempt 1 was a syntax error, then i = j = len(num1) - 1, len(num2) - 1 made both variables the same tuple (TypeError); the next two forgot to convert characters to int.
After that the final carry was dropped: "8" + "6" returned "4". The next version appended total instead of the carry temp, returning "44". Appending temp passed.

Solution
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class Solution:
    def addStrings(self, num1: str, num2: str) -> str:
        i, j, carry = len(num1) - 1, len(num2) - 1, 0
        res = []
        while i >= 0 or j >= 0 or carry:
            a = int(num1[i]) if i >= 0 else 0
            b = int(num2[j]) if j >= 0 else 0
            carry, d = divmod(a + b + carry, 10)
            res.append(str(d))
            i, j = i - 1, j - 1
        return "".join(reversed(res))

Complexity
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Time O(max(n, m)), space O(max(n, m)).

Mistakes to Avoid
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Append the final carry: "8" + "6" is "14".
Append the carry, not the last total.
i = j = a, b makes both variables tuples.

Interview Explanation
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Two pointers add digits from the ends, treating a finished string as 0 and tracking the carry; putting the carry in the loop condition keeps the final one, and I reverse the result. Time O(max(n, m)).