Hi, here's your problem today. This problem was recently asked by Facebook:

Given a list of numbers, where every number shows up twice except for one number, find that one number.

Example:

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

Here's the function signature:

```
def singleNumber(nums):
# Fill this in.
print singleNumber([4, 3, 2, 4, 1, 3, 2])
# 1
```

Challenge: Find a way to do this using O(1) memory.