+
4Sum Better
+
import java.util.*;
+
+ public class FourSumBetter {
+ public List> fourSum(int[] nums, int target) {
+ int n = nums.length;
+ Set> set = new HashSet<>();
+ for (int i = 0; i < n; i++) {
+ for (int j = i + 1; j < n; j++) {
+ HashSet seen = new HashSet<>();
+ for (int k = j + 1; k < n; k++) {
+ long sum = (long) nums[i] + nums[j] + nums[k];
+ long needed = (long) target - sum;
+ if (seen.contains(needed)) {
+ List temp = Arrays.asList(nums[i], nums[j], nums[k], (int) needed);
+ Collections.sort(temp);
+ set.add(temp);
+ }
+ seen.add((long) nums[k]);
+ }
+ }
+ }
+ return new ArrayList<>(set);
+ }
+
+ public static void main(String[] args) {
+ FourSumBetter sol = new FourSumBetter();
+ int[] nums = {1, 0, -1, 0, -2, 2}; // Example input
+ int target = 0;
+ List> result = sol.fourSum(nums, target);
+ System.out.println("Quadruplets that sum to target: " + result);
+ }
+ }
+
+
+
4Sum Optimal
+
import java.util.*;
+
+ public class FourSumOptimal {
+ public List> fourSum(int[] nums, int target) {
+ int n = nums.length;
+ List> ans = new ArrayList<>();
+ Arrays.sort(nums);
+ for (int i = 0; i < n - 3; i++) {
+ if (i > 0 && nums[i] == nums[i - 1]) continue;
+ for (int j = i + 1; j < n - 2; j++) {
+ if (j > i + 1 && nums[j] == nums[j - 1]) continue;
+ int k = j + 1;
+ int l = n - 1;
+ while (k < l) {
+ long sum = (long) nums[i] + nums[j] + nums[k] + nums[l];
+ if (sum == target) {
+ ans.add(Arrays.asList(nums[i], nums[j], nums[k], nums[l]));
+ while (k < l && nums[k] == nums[k + 1]) k++;
+ while (k < l && nums[l] == nums[l - 1]) l--;
+ k++;
+ l--;
+ }
+ else if (sum < target) {
+ k++;
+ }
+ else {
+ l--;
+ }
+ }
+ }
+ }
+ return ans;
+ }
+
+ public static void main(String[] args) {
+ FourSumOptimal sol = new FourSumOptimal();
+ int[] nums = {1, 0, -1, 0, -2, 2}; // Example input
+ int target = 0;
+ List> result = sol.fourSum(nums, target);
+ System.out.println("Quadruplets that sum to target: " + result);
+ }
+ }
+
3Sum Brute
import java.util.*;