查找表_leetcode18

# 解题思路：查找表 20190302 找工作期间
# 哈希表能将查找的复杂度从o（n）降到 o(1)
class Solution(object):
    def fourSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[List[int]]
        """

        ln = len(nums)
        res = set()
        dict = {}
        if ln < 4: return []
        nums.sort()
        for p in range(ln):
            for q in range(p + 1, ln):
                if nums[p] + nums[q] not in dict:
                    dict[nums[p] + nums[q]] = [(p, q)]
                else:
                    dict[nums[p] + nums[q]].append((p, q))

        for i in range(ln):
            for j in xrange(i + 1, ln - 2):
                T = target - nums[i] - nums[j]
                if T in dict:
                    for k in dict[T]:
                        if k[0] > j:
                            res.add((nums[i], nums[j], nums[k[0]], nums[k[1]]))

        return [list(i) for i in res]