Combination Sum II

Given a collection of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.
Each number in C may only be used once in the combination.
Solution:  Use set to handle duplicates.
Better idea is to use a previous number to mark whether duplicates exist. The following solution works for num = [1, 1] and target = 2; since each time we call dfs() and the target is still great than 0, the previous is set to be -1. But if the target is less or equal to 0, we will not reset previous to -1, and the duplicates can be detected.

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