从LeetCode用Java实现4sum

这是一个O（n ^ 3）解决方案

import java.util.Arrays;import java.util.ArrayList;import java.util.HashSet;public class Solution {public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {    // Start typing your Java solution below    // DO NOT write main() function    Arrays.sort(num);    HashSet<ArrayList<Integer>> hSet = new HashSet<ArrayList<Integer>>();    ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();    for (int i = 0; i < num.length; i++) {        for (int j = i + 1; j < num.length; j++) { for (int k = j + 1, l = num.length - 1; k < l;) {     int sum = num[i] + num[j] + num[k] + num[l];     if (sum > target) {         l--;     }     else if (sum < target) {         k++;     }     else if (sum == target) {         ArrayList<Integer> found = new ArrayList<Integer>();         found.add(num[i]);         found.add(num[j]);         found.add(num[k]);         found.add(num[l]);         if (!hSet.contains(found)) {  hSet.add(found);  result.add(found);         }         k++;         l--;     } }        }    }    return result;}

}