90. Subsets II

https://leetcode.com/problems/subsets-ii/

Given a collection of integers that might contain duplicates, nums, return all possible subsets (the power set).

Note: The solution set must not contain duplicate subsets.

Example:

Input: [1,2,2]
Output:
[
  [2],
  [1],
  [1,2,2],
  [2,2],
  [1,2],
  []
]

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Intuition
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Related problems
78-subsets
46-permutations
47-permutations-ii
39-combination-sum
40-combination-sum-ii
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