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# LC136 - Single Number

## Problem

Given a **non-empty** array of integers `nums`, every element appears *twice* except for one. Find that single one.

You must implement a solution with a linear runtime complexity and use only constant extra space.

### Example

**Input:** nums = \[2,2,1]

**Output:** 1

## Solution

### Intuition

Push elements to stack and pop when it's duplicate is found. Problem with this solution is that it does not use constant space.

### Bitwise

Bitwise XOR of two of the same number is always zero, so the remaining number after doing bitwise XOR on every element in the list is the single remaining number.

```python
def singleNumber(self, nums: List[int]) -> int:
	res = 0
	for num in nums:
		res = res ^ num
	return res
```