题解 | 三数之和

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param num int整型一维数组
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> threeSum (int[] num) {
        Arrays.sort(num);
        Set<String> set = new HashSet<String>();
        ArrayList<ArrayList<Integer>> result = new ArrayList<>();
        for (int i = 0; i < num.length - 2; i++) {
            int left = i + 1;
            int right = num.length - 1;
            while (left < right) {
                int sum = num[left] + num[i] + num[right];
                if (sum == 0) {
                    ArrayList<Integer> list = new ArrayList<>();
                    list.add(num[i]);
                    list.add(num[left]);
                    list.add(num[right]);
                    StringBuilder sb = new StringBuilder();
                    sb.append(num[i]);
                    sb.append("_");
                    sb.append(num[left]);
                    sb.append("_");
                    sb.append(num[left]);
                    if (!set.contains(sb.toString())) {
                        result.add(list);
                    }
                    set.add(sb.toString());
                    left++;
                    right--;
                } else if (sum > 0) {
                    right--;
                } else {
                    left++;
                }
            }
        }
        return result;
    }
}