Permutations.java

class Solution {
    public List<List<Integer>> permute(int[] nums) {
        ArrayList<List<Integer>> res = new ArrayList<>();
        per(nums, 0, res);
        return res;
    }

    public void per(int[] nums, int st, ArrayList<List<Integer>> res) {
        if (st == nums.length) {
            List<Integer> pr = new ArrayList<>();
            for (int i : nums) {
                pr.add(i);
            }
            res.add(pr);
            return;
        }
        for (int i = st; i < nums.length; i++) {
            swap(nums, st, i);
            per(nums, st + 1, res);
            swap(nums, st, i);
        }
    }

    public void swap(int[] nums, int i, int j) {
        int tem = nums[i];
        nums[i] = nums[j];
        nums[j] = tem;
        return;
    }
}