18 4 Sum

注意里面去重的判断： if i and nums[i] == nums[i-1], if j != i + 1 and nums[j] == nums[j - 1], while left < right and nums[left] == nums[left - 1], while left < right and nums[right] == nums[right + 1]

class Solution(object):
    def fourSum(self, nums, target):
        nums.sort()
        res = []
        length = len(nums)
        for i in range(0, length - 3):
            if i and nums[i] == nums[i - 1]:
                continue
            for j in range(i + 1, length - 2):
                if j != i + 1 and nums[j] == nums[j - 1]:
                    continue
                sum = target - nums[i] - nums[j]
                left, right = j + 1, length - 1
                while left < right:
                    if nums[left] + nums[right] == sum:
                        res.append([nums[i], nums[j], nums[left], nums[right]])
                        right -= 1
                        left += 1
                        while left < right and nums[left] == nums[left - 1]:
                            left += 1
                        while left < right and nums[right] == nums[right + 1]:
                            right -= 1
                    elif nums[left] + nums[right] > sum:
                        right -= 1
                    else:
                        left += 1
        return res