Subsets II

Like Subsets, but the array may contain duplicates — and the answer must not contain duplicate subsets.

Input:

• line 1: n
• line 2: n integers

Output: every distinct subset, one per line, in the form [a b c] (the empty subset is []): numbers inside a subset in increasing order, subsets in lexicographic order (a prefix comes first), exactly as in Subsets.

Constraints: 1 ≤ n ≤ 12, −10 ≤ value ≤ 10.

Think about it: sort, then in the backtracking loop skip a value that equals the previous one at the same depth (k > start and a[k] == a[k−1]). That one line removes every duplicate without a set.