18 4Sum

class Solution:    # @param {integer[]} nums    # @param {integer} target    # @return {integer[][]}    def fourSum(self, nums, target):        "if we adopt left-><-right method, the time complexity is O(n**3) which is not so perfect for this problem written in python(http://www.cnblogs.com/zuoyuan/p/3699384.html), thus, we can sacrifice the space for time,O(n**2)"                dic,collection={},set()        l=len(nums)        nums.sort()        if l<4:            return []        for i in range(l-1):            for j in range(i+1,l):                if nums[i]+nums[j] not in dic:                    dic[nums[i]+nums[j]]=[(i,j)]                else:                    dic[nums[i]+nums[j]].append((i,j))                for k in range(l-1):            for m in range(k+1,l):                tmp=target-nums[k]-nums[m]                if tmp in dic:                    for n in dic[tmp]:                        if n[0]>m:                            collection.add((nums[k],nums[m],nums[n[0]],nums[n[1]]))        return [list(p) for p in collection]