题目：

题解：

func fourSum(nums []int, target int) (quadruplets [][]int) {sort.Ints(nums)n := len(nums)for i := 0; i < n-3 && nums[i]+nums[i+1]+nums[i+2]+nums[i+3] <= target; i++ {if i > 0 && nums[i] == nums[i-1] || nums[i]+nums[n-3]+nums[n-2]+nums[n-1] < target {continue}for j := i + 1; j < n-2 && nums[i]+nums[j]+nums[j+1]+nums[j+2] <= target; j++ {if j > i+1 && nums[j] == nums[j-1] || nums[i]+nums[j]+nums[n-2]+nums[n-1] < target {continue}for left, right := j+1, n-1; left < right; {if sum := nums[i] + nums[j] + nums[left] + nums[right]; sum == target {quadruplets = append(quadruplets, []int{nums[i], nums[j], nums[left], nums[right]})for left++; left < right && nums[left] == nums[left-1]; left++ {}for right--; left < right && nums[right] == nums[right+1]; right-- {}} else if sum < target {left++} else {right--}}}}return
}