18. 四数之和

思路：依旧是双指针法，多一层循环

# 双指针法
class Solution:
def fourSum(self, nums: List[int], target: int) -> List[List[int]]:

nums.sort()
n = len(nums)
res = []
for i in range(n):
if i > 0 and nums[i] == nums[i - 1]: continue
for k in range(i+1, n):
if k > i + 1 and nums[k] == nums[k-1]: continue
p = k + 1
q = n - 1

while p < q:
if nums[i] + nums[k] + nums[p] + nums[q] > target: q -= 1
elif nums[i] + nums[k] + nums[p] + nums[q] < target: p += 1
else:
res.append([nums[i], nums[k], nums[p], nums[q]])
while p < q and nums[p] == nums[p + 1]: p += 1
while p < q and nums[q] == nums[q - 1]: q -= 1
p += 1
q -= 1
return res