LeetCode Q46 Permutations

Given a collection of distinct numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

Easy question, but should be careful when it comes to a solution using recursion. 

Two solutions.

• Using recursion

Using recursion, every time just insert the current element to the head of the results returned from subproblems. Otherwise there are redundancies. 

• Not using recursion

Building up the results by insert element one by one.