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]