46. 全排列

输入: [1,2,3]
输出:
[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]

class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> list = new ArrayList<>();
        dfs(res, list, nums);
        return res;
    }
    private void dfs(List<List<Integer>> res, List<Integer> list, int[] nums) {
        if (list.size() == nums.length) {
            res.add(new ArrayList<>(list));
        } else {
            for (int i = 0; i < nums.length; i++) {
                if (list.contains(nums[i])) {
                    continue;
                }
                list.add(nums[i]);
                dfs(res, list, nums);
                list.remove(list.size() - 1);
            }
        }
    }
}