# Problem Definition

## Description

Given an array of integers, every element appears thrice except for one which occurs once.

Find that element which does not appear thrice.

Note: Your algorithm should have a linear runtime complexity.

Could you implement it without using extra memory?

Example :

```plaintext
Input : [1, 2, 4, 3, 3, 2, 2, 3, 1, 1]
Output : 4
```

## Notes

1. [LeetCode discussion](https://leetcode.com/problems/single-number/discuss/42997)
1. [GeeksForGeeks](https://www.geeksforgeeks.org/find-the-element-that-appears-once/)