[leetcode]Permutations II @ Python

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