### LeetCode - 4Sum

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)

`package sum;import java.util.ArrayList;import java.util.Arrays;import java.util.List;public class FourSum {    public List<List<Integer>> fourSum(int[] nums, int target) {        int len;        List<List<Integer>> res = new ArrayList<List<Integer>>();        if (nums == null || (len = nums.length) < 4) return res;        Arrays.sort(nums);        for (int i = 0; i < len; ++i) {            List<Integer> subRes = new ArrayList<Integer>();            subRes.add(nums[i]);            getOtherThree(res, subRes, nums, i + 1, len, target);            while (i + 1 < len && nums[i + 1] == nums[i]) ++i;        }                return res;    }        private void getOtherThree(List<List<Integer>> res, List<Integer> subRes, int[] nums, int start, int len, int target) {        for (int i = start; i < len; ++i) {            List<Integer> copySubRes = new ArrayList<Integer>(subRes);            copySubRes.add(nums[i]);            getOtherTwo(res, copySubRes, nums, i + 1, len, target);            while (i + 1 < len && nums[i + 1] == nums[i]) ++i;        }    }        private void getOtherTwo(List<List<Integer>> res, List<Integer> subRes, int[] nums, int start, int len, int target) {        int sum = 0;        for (int i : subRes) sum += i;        int rem = target - sum;                int left = start;        int right = len - 1;        while (left < right) {            if (nums[left] + nums[right] == rem) {                List<Integer> copySubRes = new ArrayList<Integer>(subRes);                copySubRes.add(nums[left]);                copySubRes.add(nums[right]);                res.add(copySubRes);                do { ++left; } while (left < len && nums[left - 1] == nums[left]);                do { --right; } while (right >=0 && nums[right + 1] == nums[right]);            } else if (nums[left] + nums[right] < rem) {                ++left;            } else {                --right;            }        }    }        public static void main(String[] args) {        // TODO Auto-generated method stub        int[] nums = { 0,0,4,-2,-3,-2,-2,-3 };        FourSum f = new FourSum();        List<List<Integer>> res = f.fourSum(nums, -1);        for (List<Integer> l : res) {            for (int i : l)                System.out.print(i + "\t");            System.out.println();        }    }}`