LeetCode 3731: Find Missing Elements

Meta Data

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Difficulty: easy First Attempt: 2025-11-02

• Submissions: Accepted (first try)

Approach

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The original range is [min, max], and both ends are still in the array. Enumerate every integer in the range; those not in the array are missing, and enumerating in order keeps them sorted.

Use a set for lookups (Hash Table Template 1).

My Solution

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I originally used i not in nums, an O(n) list lookup each time; the small range let it pass. With a set:

class Solution:
    def findMissingElements(self, nums: List[int]) -> List[int]:
        seen = set(nums)
        return [x for x in range(min(nums), max(nums)) if x not in seen]

max is always present, so excluding it from the range is fine.

Complexity

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Time O(n + R) with R = max - min, space O(n).

Mistakes to Avoid

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• in on a list is a linear search; switch to a set for larger ranges.

Interview Explanation

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The minimum and maximum are still present, so I convert the array to a set and enumerate every integer between them; the ones not in the set are missing. Time O(n + R).