47. Permutations II

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

Given a collection of numbers that might contain duplicates, return all possible unique permutations.
Example:
Input: [1,1,2]
Output:
[
  [1,1,2],
  [1,2,1],
  [2,1,1]
]
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Related problems
46-permutations
78-subsets
90-subsets-ii
39-combination-sum
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