18. 4Sum.md

18. 4Sum

https://leetcode.com/problems/4sum/description/

solution

• 如何分别对四个指针去重：i,j,l,r

class Solution:
    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
        if len(nums) < 4:
            return []
        
        nums.sort()
        res = []

        for i in range(0, len(nums)):
            if i > 0 and nums[i] == nums[i-1]:
                continue
            for j in range(i+1, len(nums)):
                if j > i+1 and nums[j] == nums[j-1]:
                    continue
                
                l = j + 1
                r = len(nums) - 1

                while l < r:
                    total_sum = nums[i] + nums[j] + nums[l] + nums[r]
                    if total_sum == target:
                        res.append([nums[i], nums[j], nums[l], nums[r]])
                        l += 1
                        r -= 1

                        while l < r and nums[l] == nums[l-1]:
                            l += 1  # 这里的去重不是continue
                        while l < r and nums[r] == nums[r+1]:
                            r -= 1
                    elif total_sum > target:
                        r -= 1
                    else:
                        l += 1
        return res

时间复杂度：O(n^3)
空间复杂度：O(1)