90 LeetCode Online Judge 题目C# 练习

Given a collection of integers that might contain duplicates, S, return all possible subsets.
Note:
Elements in a subset must be in non-descending order.
The solution set must not contain duplicate subsets.
For example,
If S = [1,2,2], a solution is:

[
 [2],
 [1],
 [1,2,2],
 [2,2],
 [1,2],
 []
]

 1         public static List<List<int>> SubsetsII2(List<int> S)
 2         {
 3             S.Sort();
 4             List<List<int>> ret = new List<List<int>>();
 5 
 6             List<int> empty = new List<int>();
 7             ret.Add(empty);
 8 
 9             int start = 0;
10             for (int i = 0; i < S.Count; i++)
11             {
12                 int prev_count = ret.Count;
13                 for (int j = start; j < prev_count; j++)
14                 {
15                     List<int> temp = new List<int>(ret[j]);
16                     temp.Add(S[i]);
17                     ret.Add(temp);
18                 }
19 
20                 if ((i < S.Count - 1) && (S[i + 1] == S[i]))
21                     start = prev_count;
22                 else
23                     start = 0;
24             }
25 
26             return ret;
27         }

代码分析：

　　比Subsets那题多一个start变量，如果后一个元素跟当天这个相同，下次loop前面的时候就不要从头开始，从prev_count（上一次开始插入的位置）开始。当然也有递归的做法，不贴了。Combinatoric的做法就不适合。