[LeetCode] 90. Subsets II

Given a collection of integers that might contain duplicates, nums, return all possible subsets (the power set).

Note: The solution set must not contain duplicate subsets.

Example:

Input: [1,2,2]
Output:
[
  [2],
  [1],
  [1,2,2],
  [2,2],
  [1,2],
  []
]

子集II。

题意跟78题几乎一样，唯一多一个条件是input里面有重复元素，但是请不要重复输出相同的子集。思路还是回溯，套用78题的模板。注意这个题是需要对input排序的，在helper函数中，需要skip相同元素。

时间O(nlogn) - 因为sort了input + O(2^n) = O(2^n)

空间O(n)

Java实现

class Solution {
    public List<List<Integer>> subsetsWithDup(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        // corner case
        if (nums == null || nums.length == 0) {
            return res;
        }
        Arrays.sort(nums);
        helper(res, new ArrayList<>(), nums, 0);
        return res;
    }

    private void helper(List<List<Integer>> res, List<Integer> list, int[] nums, int start) {
        res.add(new ArrayList<>(list));
        for (int i = start; i < nums.length; i++) {
            if (i != start && nums[i] == nums[i - 1])
                continue;
            list.add(nums[i]);
            helper(res, list, nums, i + 1);
            list.remove(list.size() - 1);
        }
    }
}

LeetCode 题目总结