Cacher is the code snippet organizer for pro developers

/**
 * Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? 
 * Find all unique quadruplets in the array which gives the sum of target.
 * Note:
 *      Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
 *      The solution set must not contain duplicate quadruplets.
 * For example, given array S = {1 0 -1 0 -2 2}, and target = 0.

    A solution set is:
    (-1,  0, 0, 1)
    (-2, -1, 1, 2)
    (-2,  0, 0, 2)
 */

public class Solution {
    public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        Arrays.sort(num);
        int length = num.length;
        for(int i = 0; i < length - 3; i++){
            if(i > 0 && num[i] == num[i - 1])
                continue;
            for(int j = i + 1; j < length - 2; j++){
                if(j > i + 1 && num[j] == num[j - 1])
                    continue;
                int l = j + 1;
                int r = length - 1;
                while(l < r){
                    int delta = num[i] + num[j] + num[l] + num[r] - target;
                    if(delta == 0){
                        // skip duplicates
                        if(l > j + 1 && num[l] == num[l - 1]){
                            l++;
                            continue;
                        }
                        if(r < length - 1 && num[r] == num[r + 1]){
                            r--;
                            continue;
                        }
                        ArrayList<Integer> item = new ArrayList<Integer>();
                        item.add(num[i]);
                        item.add(num[j]);
                        item.add(num[l]);
                        item.add(num[r]);
                        res.add(item);
                        l++;
                        r--;
                    } else if (delta < 0) 
                        l++;
                     else 
                        r--;
                }
            }
        }
        return res;
    }
}