Update solution 0018

leetcode/0018.4Sum/18. 4Sum.go

@@ -2,7 +2,92 @@ package leetcode
 
 import "sort"
 
-func fourSum(nums []int, target int) [][]int {
+// 解法一 双指针
+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
+}
+
+// 解法二 kSum
+func fourSum1(nums []int, target int) [][]int {
+	res, cur := make([][]int, 0), make([]int, 0)
+	sort.Ints(nums)
+	kSum(nums, 0, len(nums)-1, target, 4, cur, &res)
+	return res
+}
+
+func kSum(nums []int, left, right int, target int, k int, cur []int, res *[][]int) {
+	if right-left+1 < k || k < 2 || target < nums[left]*k || target > nums[right]*k {
+		return
+	}
+	if k == 2 {
+		// 2 sum
+		twoSum(nums, left, right, target, cur, res)
+	} else {
+		for i := left; i < len(nums); i++ {
+			if i == left || (i > left && nums[i-1] != nums[i]) {
+				next := make([]int, len(cur))
+				copy(next, cur)
+				next = append(next, nums[i])
+				kSum(nums, i+1, len(nums)-1, target-nums[i], k-1, next, res)
+			}
+		}
+	}
+
+}
+
+func twoSum(nums []int, left, right int, target int, cur []int, res *[][]int) {
+	for left < right {
+		sum := nums[left] + nums[right]
+		if sum == target {
+			cur = append(cur, nums[left], nums[right])
+			temp := make([]int, len(cur))
+			copy(temp, cur)
+			*res = append(*res, temp)
+			// reset cur to previous state
+			cur = cur[:len(cur)-2]
+			left++
+			right--
+			for left < right && nums[left] == nums[left-1] {
+				left++
+			}
+			for left < right && nums[right] == nums[right+1] {
+				right--
+			}
+		} else if sum < target {
+			left++
+		} else {
+			right--
+		}
+	}
+}
+
+// 解法三
+func fourSum2(nums []int, target int) [][]int {
 	res := [][]int{}
 	counter := map[int]int{}
 	for _, value := range nums {