Restore an Array with One Displaced Element
Last updated: July 2, 2026
Quick Overview
Given an array that contains all but one of the elements in a sequence, the candidate is asked to restore the original array by identifying the missing element.
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July 2, 20264
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Given an array that contains all but one of the elements in a sequence, the candidate is asked to restore the original array by identifying the missing element.
This question is typically asked during the technical screen to evaluate a candidate's problem-solving skills and ability to work with algorithms. The interviewer is looking for a clear understanding of array manipulation, efficient algorithm design, and the ability to articulate the thought process behind the solution.
What the Interviewer Expects
- Clearly explain the approach to identify the missing element.
- Provide an efficient algorithm with a focus on time and space complexity.
- Include a working code example in Python.
- Discuss edge cases and how they are handled.
- Articulate the reasoning behind the chosen solution.
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- How would your approach change if the array had multiple missing elements?
- Can you explain the time and space complexity of your solution?
- What edge cases did you consider while implementing your solution?
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Practice DSA ProblemsSample Answer
Approach
To restore the original array, we need to identify the missing element. A common approach is to calculate the expected sum of the original array and subtract the sum of the provided array. The differe...
Algorithm
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Calculate the expected sum of the original array using the formula for the sum of the first n natural numbers:
expected_sum = n * (n + 1) / 2
where n is the length of the or...
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