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

題意：

Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,
[1,1,2] have the following unique permutations:
[1,1,2], [1,2,1], and [2,1,1].

解題思路：這道題也是窮舉全排列，只是集合中可能有重複的元素。分兩步：1，對集合進行排序。2，進行剪枝，如果元素重複，直接跳過這一元素，決策樹的這一枝被剪掉。

程式碼：

class Solution:

    # @param num, a list of integer

    # @return a list of lists of integers

    def permuteUnique(self, num):

        length = len(num)

        if length == 0: return []

        if length == 1: return [num]

        num.sort()

        res = []

        previousNum = None

        for i in range(length):

            if num[i] == previousNum: continue

            previousNum = num[i]

            for j in self.permuteUnique(num[:i] + num[i+1:]):

                res.append([num[i]] + j)

        return res