暴力解法import java.util.*;public class _15_ThreeSum1 {public List<List<Integer>> threeSum(int[] nums) {if (nums == null || nums.length < 3)return new ArrayList<>();Set<List<Integer>> res = new HashSet<>();Arrays.sort(nums); // O(nlogn)// O(n^3)for (int i = 0; i < nums.length; i++) {for (int j = i + 1; j < nums.length; j++) {for (int k = j + 1; k < nums.length; k++) {if (nums[i] + nums[j] + nums[k] == 0) {res.add(Arrays.asList(nums[i], nums[j], nums[k]));}}}}return new ArrayList<>(res);}public static void main(String[] args) {int[] nums = {-1, 0, 1, 2, -1, -4};System.out.println(new _15_ThreeSum1().threeSum(nums));}
}

import java.util.*;public class _15_ThreeSum2 {public List<List<Integer>> threeSum(int[] nums) {if (nums == null || nums.length < 3)return new ArrayList<>();Set<List<Integer>> res = new HashSet<>();Arrays.sort(nums); // O(nlogn)// O(n^2)for (int i = 0; i < nums.length; i++) {// 在 i + 1 ... nums.length - 1 中查找相加等于 -nums[i] 的两个数int target = -nums[i];int left = i + 1;int right = nums.length - 1;while (left < right) {int sum = nums[left] + nums[right];if (sum == target) {res.add(Arrays.asList(nums[i], nums[left], nums[right]));left++;right--;} else if (sum < target) {left++;} else {right--;}}}return new ArrayList<>(res); // O(n)}public static void main(String[] args) {int[] nums = {-1, 0, 1, 2, -1, -4};System.out.println(new _15_ThreeSum2().threeSum(nums));}
}

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;public class _15_ThreeSum3 {public List<List<Integer>> threeSum(int[] nums) {if (nums == null || nums.length < 3)return new ArrayList<>();List<List<Integer>> res = new ArrayList<>();Arrays.sort(nums); // O(nlogn)// O(n^2)for (int i = 0; i < nums.length - 2; i++) {if (i > 0 && nums[i] == nums[i - 1]) continue;// 在 i + 1 ... nums.length - 1 中查找相加等于 -nums[i] 的两个数int target = -nums[i];int left = i + 1;int right = nums.length - 1;while (left < right) {int sum = nums[left] + nums[right];if (sum == target) {res.add(Arrays.asList(nums[i], nums[left], nums[right]));/*下面的代码相当于：while (left < right) {// 不管前后相不相等，left 都要往前走left++;if (nums[left - 1] != nums[left]) break;}while (left < right) {// 不管前后相不相等，right 都要往后走right--;if (nums[right + 1] != nums[right]) break;}*/// 去重while (left < right && nums[left] == nums[++left]);while (left < right && nums[right] == nums[--right]);} else if (sum < target) {left++;} else {right--;}}}return res;}public static void main(String[] args) {int[] nums = {-1, 0, 1, 2, -1, -4};System.out.println(new _15_ThreeSum3().threeSum(nums));}
}