LeetCode [90] Subsets II

Note:

• eg: all the subsets of [1,2,2,3]
• subset of size 0: []
• subset of size 1: [1] [2] [2 ] [3] 
• subset of size 2: [1,2] [1,2] [1,3]     [2,2] [2,3]   
• subset of size 3: [1,2,2] [1,2,3]     [2,2,3]
• repeatedly add one succeeding number (if it is not repeating) to each element of the previous set
• avoid duplicates by comparing successive numbers

Ref
[1] https://leetcode.com/problems/subsets-ii/
OJ