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Problem15_3Sum.java
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package com.longluo.top100;
import java.util.*;
/**
* 15. 三数之和
* <p>
* 给你一个包含 n 个整数的数组 nums，判断 nums 中是否存在三个元素 a，b，c ，使得 a + b + c = 0 ？请你找出所有和为 0 且不重复的三元组。
* 注意：答案中不可以包含重复的三元组。
* <p>
* 示例 1：
* 输入：nums = [-1,0,1,2,-1,-4]
* 输出：[[-1,-1,2],[-1,0,1]]
* <p>
* 示例 2：
* 输入：nums = []
* 输出：[]
* <p>
* 示例 3：
* 输入：nums = [0]
* 输出：[]
* <p>
* 提示：
* 0 <= nums.length <= 3000
* -10^5 <= nums[i] <= 10^5
* <p>
* https://leetcode.com/problems/3sum/
*/
public class Problem15_3Sum {
// Brute Force time: O(n^3) space: O(1)
// Failed for not remove the same elements.
public static List<List<Integer>> threeSum_bf(int[] nums) {
if (nums == null || nums.length < 3) {
return new ArrayList<>();
}
Set<List<Integer>> ans = new HashSet<>();
int length = nums.length;
for (int i = 0; i < length - 2; i++) {
for (int j = i + 1; j < length - 1; j++) {
for (int k = j + 1; k < length; k++) {
if (nums[i] + nums[j] + nums[k] == 0) {
ans.add(Arrays.asList(nums[i], nums[j], nums[k]));
}
}
}
}
return new ArrayList<>(ans);
}
// Sort + BF + Set time: O(n^3) space: O(1)
// TimeOut
public static List<List<Integer>> threeSum_bf_sort(int[] nums) {
if (nums == null || nums.length < 3) {
return new ArrayList<>();
}
Set<List<Integer>> ans = new HashSet<>();
Arrays.sort(nums);
int length = nums.length;
for (int i = 0; i < length - 2; i++) {
for (int j = i + 1; j < length - 1; j++) {
for (int k = j + 1; k < length; k++) {
if (nums[i] + nums[j] + nums[k] == 0) {
ans.add(Arrays.asList(nums[i], nums[j], nums[k]));
}
}
}
}
return new ArrayList<>(ans);
}
// Sort + BF + List time: O(n^3) space: O(1)
// TimeOut
public static List<List<Integer>> threeSum_bf_list(int[] nums) {
if (nums == null || nums.length < 3) {
return new ArrayList<>();
}
List<List<Integer>> ans = new ArrayList<>();
Arrays.sort(nums);
int length = nums.length;
for (int i = 0; i < length - 2; i++) {
if (nums[i] > 0) {
break;
}
if (i > 0 && nums[i] == nums[i - 1]) {
continue;
}
for (int j = i + 1; j < length - 1; j++) {
if (j > i + 1 && nums[j] == nums[j - 1]) {
continue;
}
for (int k = j + 1; k < length; k++) {
if (k > j + 1 && nums[k] == nums[k - 1]) {
continue;
}
if (nums[i] + nums[j] + nums[k] == 0) {
ans.add(Arrays.asList(nums[i], nums[j], nums[k]));
}
}
}
}
return ans;
}
// Hash time: O(n^2) space: O(n)
public static List<List<Integer>> threeSum_hash(int[] nums) {
if (nums == null || nums.length < 3) {
return new ArrayList<>();
}
Set<List<Integer>> ans = new HashSet<>();
Arrays.sort(nums);
int len = nums.length;
for (int i = 0; i < len - 2; i++) {
if (nums[i] > 0) {
break;
}
if (i > 0 && nums[i] == nums[i - 1]) {
continue;
}
int target = -nums[i];
Set<Integer> set = new HashSet<>();
for (int j = i + 1; j < len; j++) {
if (set.contains(target - nums[j])) {
ans.add(Arrays.asList(nums[i], nums[j], target - nums[j]));
} else {
set.add(nums[j]);
}
}
}
return new ArrayList<>(ans);
}
// Two Pointers time: O(n^2) space: O(1)
public static List<List<Integer>> threeSum_tp(int[] nums) {
if (nums == null || nums.length < 3) {
return new ArrayList<>();
}
List<List<Integer>> ans = new ArrayList<>();
Arrays.sort(nums);
int length = nums.length;
for (int i = 0; i < length - 2; i++) {
if (nums[i] > 0) {
break;
}
if (i > 0 && nums[i] == nums[i - 1]) {
continue;
}
if (nums[i] + nums[length - 2] + nums[length - 1] < 0) {
continue;
}
int left = i + 1;
int right = length - 1;
while (left < right) {
if (nums[left] + nums[right] < -nums[i]) {
left++;
} else if (nums[left] + nums[right] > -nums[i]) {
right--;
} else if (nums[left] + nums[right] == -nums[i]) {
ans.add(Arrays.asList(nums[i], nums[left], nums[right]));
left++;
right--;
while (left < right && nums[left] == nums[left - 1]) {
left++;
}
while (left < right && nums[right] == nums[right + 1]) {
right--;
}
}
}
}
return ans;
}
public static void main(String[] args) {
System.out.println("[[-1,-1,2],[-1,0,1]] ?= " + threeSum_bf(new int[]{-1, 0, 1, 2, -1, -4}));
System.out.println("[[-1,-1,2],[-1,0,1]] ?= " + threeSum_bf_sort(new int[]{-1, 0, 1, 2, -1, -4}));
System.out.println("[[-1,-1,2],[-1,0,1]] ?= " + threeSum_bf_list(new int[]{-1, 0, 1, 2, -1, -4}));
System.out.println("[[-1,-1,2],[-1,0,1]] ?= " + threeSum_hash(new int[]{-1, 0, 1, 2, -1, -4}));
System.out.println("[0, 0, 0] ?= " + threeSum_bf(new int[]{0, 0, 0, 0}));
System.out.println("[0, 0, 0] ?= " + threeSum_bf_list(new int[]{0, 0, 0, 0}));
System.out.println("[[-1,-1,2],[-1,0,1]] ?= " + threeSum_tp(new int[]{-1, 0, 1, 2, -1, -4}));
System.out.println("[0, 0, 0] ?= " + threeSum_tp(new int[]{0, 0, 0, 0}));
}
}