4Sum

//from 3Sum, and 3Sum is from 2Sum
public class Solution {
    public List<List<Integer>> fourSum(int[] num, int target) {
        Arrays.sort(num);
        List<List<Integer>> res = new LinkedList<>();
        for (int j = 0; j < num.length-3; j++) {
            if (j == 0 || (j > 0 && num[j] != num[j-1])) {
                for (int i = j+1; i < num.length-2; i++) {
                    if (i == j+1 || (i > j+1 && num[i] != num[i-1])) {
                        int lo = i+1, hi = num.length-1, sum = target - num[j] - num[i];
                        while (lo < hi) {
                            if (num[lo] + num[hi] == sum) {
                                res.add(Arrays.asList(num[j] ,num[i], num[lo], num[hi]));
                                while (lo < hi && num[lo] == num[lo+1]) lo++;
                                while (lo < hi && num[hi] == num[hi-1]) hi--;
                                lo++; hi--;
                            } else if (num[lo] + num[hi] < sum) lo++;
                            else hi--;
                       }
                    }
                }
            }
        }
        return res;
    }
}