Permutations.java

class Solution {
    public List<List<Integer>> permute(int[] nums) {

        List<List<Integer>> result = new ArrayList<>();
        if(nums == null || nums.length == 0) return result;

        List<Integer> listNums = new ArrayList<>(nums.length);
        for (int i : nums) { listNums.add(i); }

        permute(listNums, 0, nums.length, result);

        return result;
    }

    void permute(List<Integer> nums, int left, int right, List<List<Integer>> result){
        if(left == right){
            result.add(new ArrayList<>(nums));
        }
        for (int i = left; i < right; i++) {
            swap(nums, left, i);
            permute(nums, left+1, right, result);
            swap(nums, left, i);
        }
    }

    void swap(List<Integer> nums, int left, int right){
        int temp = nums.get(left);
        nums.set(left, nums.get(right));
        nums.set(right, temp);
    }
}