[leetcode]Subsets II @ Python

原题地址：https://oj.leetcode.com/problems/subsets-ii/

题意：

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],

  []

]

解题思路：和上一道题一样，求一个集合的所有子集。和上一道题不一样的一点是集合可能有重复元素。这道题同样使用dfs来解题，只是需要在dfs函数里加一个剪枝的条件，排除掉同样的子集。

代码：

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

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