leetcode46. 全排列

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