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]
]
List<Integer>的大小是size()
permutations,remove的是最后一位。(permutations.size() - 1);
results.add(new ArrayList<>(permutations));