package leetcode;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

/**
 * https://leetcode.com/problems/4sum/
 */
public class Problem18 {
    public List<List<Integer>> fourSum(int[] num, int target) {
        List<List<Integer>> result = new ArrayList<>();
        Map<Integer, Integer> map = new HashMap<>();
        Arrays.sort(num);
        for (int i = 0; i < num.length; i++) {
            map.put(num[i], i);
        }
        Set<String> set = new HashSet<>();
        for (int i = 0; i < num.length; i++) {
            for (int j = i+1; j < num.length; j++) {
                for (int k = j+1; k < num.length; k++) {
                    int expected = target - (num[i] + num[j] + num[k]);
                    if (map.containsKey(expected)) {
                        int l = map.get(expected);
                        if (l > k) {
                            List<Integer> list = Arrays.asList(num[i], num[j], num[k], expected);
                            if (!set.contains(list.toString())) {
                                result.add(list);
                            }
                            set.add(list.toString());
                        }
                    }
                }
            }
        }
        return result;
    }
    
    public static void main(String[] args) {
        Problem18 prob = new Problem18();
        List<List<Integer>> result = prob.fourSum(new int[]{1, 0, -1, 0, -2, 2}, 0);
        for (List<Integer> list : result) {
            System.out.println(list);
        }
        result = prob.fourSum(new int[]{91277418,66271374,38763793,4092006,11415077,60468277,1122637,72398035,-62267800,22082642,60359529,-16540633,92671879,-64462734,-55855043,-40899846,88007957,-57387813,-49552230,-96789394,18318594,-3246760,-44346548,-21370279,42493875,25185969,83216261,-70078020,-53687927,-76072023,-65863359,-61708176,-29175835,85675811,-80575807,-92211746,44755622,-23368379,23619674,-749263,-40707953,-68966953,72694581,-52328726,-78618474,40958224,-2921736,-55902268,-74278762,63342010,29076029,58781716,56045007,-67966567,-79405127,-45778231,-47167435,1586413,-58822903,-51277270,87348634,-86955956,-47418266,74884315,-36952674,-29067969,-98812826,-44893101,-22516153,-34522513,34091871,-79583480,47562301,6154068,87601405,-48859327,-2183204,17736781,31189878,-23814871,-35880166,39204002,93248899,-42067196,-49473145,-75235452,-61923200,64824322,-88505198,20903451,-80926102,56089387,-58094433,37743524,-71480010,-14975982,19473982,47085913,-90793462,-33520678,70775566,-76347995,-16091435,94700640,17183454,85735982,90399615,-86251609,-68167910,-95327478,90586275,-99524469,16999817,27815883,-88279865,53092631,75125438,44270568,-23129316,-846252,-59608044,90938699,80923976,3534451,6218186,41256179,-9165388,-11897463,92423776,-38991231,-6082654,92275443,74040861,77457712,-80549965,-42515693,69918944,-95198414,15677446,-52451179,-50111167,-23732840,39520751,-90474508,-27860023,65164540,26582346,-20183515,99018741,-2826130,-28461563,-24759460,-83828963,-1739800,71207113,26434787,52931083,-33111208,38314304,-29429107,-5567826,-5149750,9582750,85289753,75490866,-93202942,-85974081,7365682,-42953023,21825824,68329208,-87994788,3460985,18744871,-49724457,-12982362,-47800372,39958829,-95981751,-71017359,-18397211,27941418,-34699076,74174334,96928957,44328607,49293516,-39034828,5945763,-47046163,10986423,63478877,30677010,-21202664,-86235407,3164123,8956697,-9003909,-18929014,-73824245}, -236727523);
        for (List<Integer> list : result) {
            System.out.println(list);
        }
    }
}

Write something for problem 18