public List<List<Integer>> fourSum(int[] nums, int target) {
        if (nums.length < 4) {
            return Collections.EMPTY_LIST;
        }
        Arrays.sort(nums);
        List<List<Integer>> result = new ArrayList<>();
        for (int i = 0; i < nums.length - 3; i++) {
            if (i != 0 && nums[i] == nums[i - 1]) {
                continue;
            }
            if(nums[i] + nums[i+1] + nums[i+2] + nums[i+3] > target) {
                break;
            }
            if(nums[i] + nums[nums.length - 3] + nums[nums.length - 2] + nums[nums.length - 1] < target) {
                continue;
            }
            for (int j = i + 1; j < nums.length - 2; j++) {
                if (j != i + 1 && nums[j] == nums[j - 1]) {
                    continue;
                }
                if (nums[i] + nums[j] + nums[j + 1] + nums[j + 2] > target) {
                    break;
                }
                if (nums[i] + nums[j] + nums[nums.length - 1] + nums[nums.length - 1] < target) {
                    continue;
                }
                int low = j + 1;
                int high = nums.length - 1;
                while (low < high) {
                    if (low != j + 1 && nums[low] == nums[low - 1]) {
                        low++;
                        continue;
                    }
                    if (high != nums.length - 1 && nums[high] == nums[high + 1]) {
                        high--;
                        continue;
                    }

                    int sum = nums[i] + nums[j] + nums[low] + nums[high];
                    if (sum == target) {
                        List<Integer> r = new ArrayList<>();
                        r.add(nums[i]);
                        r.add(nums[j]);
                        r.add(nums[low]);
                        r.add(nums[high]);
                        result.add(r);
                        low++;
                        high--;
                    } else if (sum < target) {
                        low++;
                    } else {
                        high--;
                    }
                }
            }
        }
        return result;
    }
Date 07/31/2016
@author Tushar Roy

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.

Time complexity O(n^3)
Space complexity O(1)

Reference
https://leetcode.com/problems/4sum/
src/com/interview/array/FourSum.java