Problem 37-triples whose sum is 0 in the array

Original title link

Expansion topic-the closest sum of three numbers

Extended question two-the sum of four numbers is a specific value

Title description

给出一个有n个元素的数组S，S中是否有元素a,b,c满足a+b+c=0？找出数组S中所有满足条件的三元组。
注意：
	1.三元组（a、b、c）中的元素必须按非降序排列。（即a≤b≤c）
	2.解集中不能包含重复的三元组。

Example

输入：
[-2,0,1,1,2]
输出：
[[-2,0,2],[-2,1,1]]

Reference solution

import java.util.ArrayList;
import java.util.Arrays;
public class Solution {
    
    
    public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
    
    
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        Arrays.sort(num);
        ArrayList<Integer> r;
        for (int i = 0; i < num.length; i++) {
    
    
            // 过滤掉相同的左侧的值
            if (i >= 1 && num[i] == num[i - 1]) 
                continue;
            for (int j = i + 1; j < num.length; j++) {
    
    
                // 过滤掉相同的中间的值
                if (j >= i + 2 && num[j] == num[j - 1]) 
                    continue;
                int rest = -(num[i] + num[j]);
                int index = Arrays.binarySearch(num, j + 1, num.length, rest);
                // 查找到会返回一个大于等于0的值
                if (index >= 0) {
    
    
                    r = new ArrayList<>();
                    r.add(num[i]);
                    r.add(num[j]);
                    r.add(num[index]);
                    res.add(r);
                }
            }
        }
        return res;
    }
}

to sum up

This topic is mainly about learning how to deal with it 三个数的关系and using it 一重循环+双指针instead 三重循环.