4Sum.java

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        Arrays.sort(nums);
        List<List<Integer>> mainList = new ArrayList<>();

        for (int i = 0; i < nums.length; i++) {
            if (i > 0 && nums[i] == nums[i - 1])
                continue;

            for (int j = i + 1; j < nums.length; j++) {
                if (j != i + 1 && nums[j] == nums[j - 1])
                    continue;

                int left = j + 1;
                int right = nums.length - 1;

                while (left < right) {

                    double val = (double)nums[i] + (double)nums[j] + (double)nums[left] + (double)nums[right];

                    if (val == target) {
                        List<Integer> list = new ArrayList<>();
                        list.add(nums[i]);
                        list.add(nums[j]);
                        list.add(nums[left]);
                        list.add(nums[right]);
                        mainList.add(list);
                        
                        while (left < right && nums[left] == nums[left + 1])
                            left++;

                        while (left < right && nums[right] == nums[right - 1])
                            right--;

                        left++;
                        right--;
                    } else if (val < target) {
                        left++;
                    } else {
                        right--;
                    }
                }
            }
        }
        return mainList;
    }
}