【LeetCode/LintCode】Facebook面试题：子集 II

            输入：[0]
输出：
[
  [],
  [0]
]
        

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

            ublic class Solution {
    /**
     * @param nums: A set of numbers.
     * @return: A list of lists. All valid subsets.
     */
    public List<List<Integer>> subsetsWithDup(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        // 排序
        Arrays.sort(nums);
        // dfs搜索
        Deque<Integer> subset = new ArrayDeque<>(nums.length);
        dfs(nums, 0, subset, res);
        return res;
    }
    private void dfs(int[] nums, int k, Deque<Integer> subset, List<List<Integer>> res) {
        // 当前组合存入res
        res.add(new ArrayList<>(subset));
        // 为subset新增一位元素
        for (int i = k; i < nums.length; ++i) {
            // 剪枝
            if (i != k && nums[i] == nums[i - 1]){
                continue;
            }
            subset.addLast(nums[i]);
            // 下一层搜索
            dfs(nums, i + 1, subset, res);
            // 回溯
            subset.removeLast();
        }
    }
}