Imagine you have a list of all the numbers from 1 to n in a list, except one of the numbers is missing, (therefore the list has (n-1) numbers), and the numbers are in a random order in the list.How do you find out which number is missing?

Answer:

```
sum(xrange(1, n+1)) - sum(list)
```

P.S. Even better:

```
n * (1 + n) / 2 - sum(list)
```