308. Smallest Window Holding Everything

Print the length of the shortest contiguous stretch that contains every distinct value present in the whole array.

1 2 2 3 1 -> 3

The array holds three distinct values, and 2 3 1 is the shortest stretch containing all of them.

Count the distinct values first, so you know what "everything" means. Then grow a window on the right, and whenever it holds them all, record its length and shrink from the left for as long as it still holds them all.

Every element enters and leaves the window once, so despite the nested loop the whole thing is linear.

Constraints - `1 ≤ n ≤ 200000` - `-1000000000 ≤ a[i] ≤ 1000000000`