排列，含重复元素和不重复元素两种情况的实现

用的典型的回溯法：

    public List> permute(int[] nums) {
        List> result = new ArrayList<>();
        permute(result, nums, 0);
        return result;
    }

    private void permute(List> result, int[] nums, int idx) {
        if (idx == nums.length) {
            ArrayList arrayList = new ArrayList<>();
            for (int num : nums) {
                arrayList.add(num);
            }
            result.add(arrayList);
        }
        for (int i = idx; i < nums.length; i++) {
            swap(nums, idx, i);
            permute(result, nums, idx + 1);
            swap(nums, idx, i);
        }
    }

    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }

    public List> permuteUnique(int[] nums) {
        List> result = new ArrayList<>();
        permuteUnique(result, nums, 0);
        return result;
    }

    private void permuteUnique(List> result, int[] nums, int idx) {
        if (idx == nums.length) {
            ArrayList arrayList = new ArrayList<>();
            for (int num : nums) {
                arrayList.add(num);
            }
            result.add(arrayList);
        }
        for (int i = idx; i < nums.length; i++) {
            if (!isDuplicate(nums, idx, i)) {
                swap(nums, idx, i);
                permuteUnique(result, nums, idx + 1);
                swap(nums, idx, i);
            }
        }
    }

    private boolean isDuplicate(int[] nums, int idx, int i) {
        for (int i1 = idx; i1 < i; i1++) {
            if (nums[i1] == nums[i]) {
                return true;
            }
        }
        return false;
    }