Is Set Mismatch easy or hard?

Set Mismatch is categorized as an Easy problem. The main challenge is recognizing that the array should contain numbers from 1 to n with exactly one duplicate and one missing value. Once that constraint is understood, frequency counting or simple math leads to an efficient solution.

How to solve Set Mismatch in O(n)?

Iterate through the array once while tracking either frequencies or mathematical sums. A HashMap can detect the duplicate immediately and then identify the missing number during a second pass from 1..n. The math-based approach compares the expected sum of numbers 1..n with the actual array sum to derive both values.

What is the best approach for Set Mismatch?

Two common approaches solve Set Mismatch in linear time. The HashMap frequency counting method is the most intuitive and easy to implement. The mathematical Sum and Difference method is more space-efficient because it runs in O(n) time with O(1) extra space.

What data structure is used in Set Mismatch?

The most common data structure used is a HashMap or hash table to store frequency counts of numbers in the array. This allows O(1) average lookup time to detect duplicates and missing elements efficiently. Some optimized solutions avoid extra data structures by using mathematical properties instead.

What is the time complexity of Set Mismatch?

Optimal solutions run in O(n) time where n is the size of the array. Each element is processed once either while building a frequency map or while computing sums and differences. Space complexity depends on the approach: O(n) for HashMap counting and O(1) for the mathematical method.

Set Mismatch Python or Java solution approach?

Python and Java implementations typically follow the same logic. Either maintain a hash map (or dictionary) to track counts, or compute mathematical differences between the expected and actual sums of numbers. Both approaches achieve O(n) time complexity.

Is Set Mismatch asked at Google, Amazon, or Meta?

Set Mismatch represents a common array and hash table pattern frequently used in technical interviews. Similar duplicate-detection problems appear in interviews at large tech companies including Amazon, Google, and Meta because they test array traversal, hashing, and reasoning about constraints.

Description Solution Video 19:43

645. Set Mismatch

Easy43.4% acceptance7 companies

You have a set of integers s, which originally contains all the numbers from 1 to n. Unfortunately, due to some error, one of the numbers in s got duplicated to another number in the set, which results in repetition of one number and loss of another number.

You are given an integer array nums representing the data status of this set after the error.

Find the number that occurs twice and the number that is missing and return them in the form of an array.

Example 1: