[leetcode]Subsets II @ Python

原題地址：https://oj.leetcode.com/problems/subsets-ii/

題意：

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],
  []
]

解題思路：和上一道題一樣，求一個集合的所有子集。和上一道題不一樣的一點是集合可能有重復元素。這道題同樣使用dfs來解題，只是需要在dfs函數里加一個剪枝的條件，排除掉同樣的子集。

代碼：

class Solution:
    # @param num, a list of integer
    # @return a list of lists of integer
    def subsetsWithDup(self, S):
        def dfs(depth, start, valuelist):
            if valuelist not in res: res.append(valuelist)
            if depth == len(S): return
            for i in range(start, len(S)):
                dfs(depth+1, i+1, valuelist+[S[i]])
        S.sort()
        res = []
        dfs(0, 0, [])
        return res