Permutations.md

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],
[3,2,1]
]

There are at least two ways to solve it.

1. DFS We use a flag array to tag the numbers that have been traversed, so we can iterate the array with dfs.
2. find the pattern. Use the idea similar to the next permutation.