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[leetcode]Permutations II @ Python
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原题地址：https://oj.leetcode.com/problems/permutations-ii/
题意：
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].
解题思路：这道题也是穷举全排列，只是集合中可能有重复的元素。分两步：1，对集合进行排序。2，进行剪枝，如果元素重复，直接跳过这一元素，决策树的这一枝被剪掉。
代码：
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
