LeetCode 3550: Smallest Index With Digit Sum Equal to Index

Meta Data

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Difficulty: easy First Attempt: 2025-05-18

• Submissions: Accepted (first try)

Approach

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Starting from index 0, compute the digit sum of nums[i]; the first one equal to i is the smallest index. If none, return -1.

My Solution

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class Solution:
    def smallestIndex(self, nums: List[int]) -> int:
        for i, x in enumerate(nums):
            if sum(int(c) for c in str(x)) == i:
                return i
        return -1

Complexity

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Time O(n · d) with d digits, space O(d).

Mistakes to Avoid

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• Return the smallest index: scan from the start and stop at the first match.

Interview Explanation

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Scanning indices in increasing order, I sum each number’s digits via its string form and return the first index that matches, otherwise -1. Time O(n · d).