Find the duplicate number-coding question asked by Meta, JPMC

2 min readJun 19, 2022


Problem statement:

Given an array of integers nums containing n + 1 integers where each integer is in the range [1, n] inclusive.

There is only one repeated number in nums, return this repeated number.

You must solve the problem without modifying the array nums and uses only constant extra space.

Input Format:

Array nums

Output Format:

Repeated number in the array

Sample Input 1:

nums = [1,3,4,2,2]

Sample Output 1:


Sample Input 2:

nums = [3,1,3,4,2]

Sample Output 2:



1 <= n <= 105nums.length == n + 11 <= nums[i] <= n


We need to insert elements into a data structure and look them up in constant time, so as to improve the complexity of sorting approach. A set can be used to satisfy these conditions. So, we iterate over the array and insert each element into a set. Before inserting we check whether the element is already present in the set or not. If it is found, we get the repeated element and hence return it.

Time complexity: O(n)

Space complexity: O(n)


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