46. Permutations

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

Given a collection of distinct integers, return all possible permutations.
Example:
Input: [1,2,3]
Output:
[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]
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Related problems
47-permutations-ii
78-subsets
90-subsets-ii
39-combination-sum
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