类似于力扣：15. 三数之和

import java.util.*;

public class Solution {
    public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        if (num == null || num.length < 3) return res;
        Arrays.sort(num);

        for (int i = 0; i < num.length; i++) {
            if (num[i] > 0) return res;
            if (i > 0 && num[i] == num[i - 1]) continue;
            int l = i + 1;
            int r = num.length - 1;

            while (l < r) {
                int sum = num[i] + num[l] + num[r];
                if (sum == 0) {
                    //强制转换类型
                    res.add(new ArrayList<>(Arrays.asList(num[i], num[l], num[r])));
                    while (l < r && num[l] == num[l + 1]) l++;
                    while (l < r && num[r] == num[r - 1]) r--;
                    l++;
                    r--;
                } 
                else if (sum < 0) l++;
                else if (sum > 0) r--;
            }
        }
        return res;
    }
}