Element that appears once

Posted: 14 Jan, 2021
Difficulty: Easy

PROBLEM STATEMENT

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You are given an arbitrary array ‘arr’ consisting of N non-negative integers, where every element appears thrice except one. You need to find the element that appears only once.

Input Format:
The first line of the input contains a single integer T, representing the number of test cases. 

The first line of each test case consists of a single integer N, representing the number of elements in the given array.

The second line of each test case contains N space-separated integers, denoting the elements of the array.
Output Format:
For each test case, print a single integer representing the element that appears only once in the array.
Note:
You do not need to print anything. It has already been taken care of. Just implement the given function. 
Constraints:
1 <= T <= 100
4 <= N <= 10^4
0 <= arr[i] < 10^9
Time Limit: 1sec
Follow Up:
Try to solve this problem in O(N) time and O(1) space complexity.
Approach 1

An easy way to solve this problem is to first store the count of all the elements in a map with keys as elements of the array and value as their frequencies. Then iterate through the map to find out the element whose frequency is 1. 

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