18. 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)

public class Solution {
    public List<List<Integer>> fourSum(int[] num, int target) {
        Arrays.sort(num);

        List<List<Integer>> list = new ArrayList<List<Integer>>();
        for (int i = 0; i <= num.length - 3; i++) {
            if (i != 0 && num[i] == num[i-1])
                continue;

            for (int j = i + 1; j <= num.length - 2; j++) {              
                if (j != i + 1 && num[j] == num[j-1])
                    continue;

                int left = j + 1, right = num.length - 1;
                while (left < right) {
                    if (left != j + 1 && num[left] == num[left-1]) {
                        left++;
                        continue;
                    }
                    if (right != num.length - 1 && num[right] == num[right+1]) {
                        right--;
                        continue;
                    }

                    int sum = num[i] + num[j] + num[left] + num[right];
                    if (sum == target) {
                        List<Integer> item = new ArrayList<Integer>();
                        item.add(num[i]);
                        item.add(num[j]);
                        item.add(num[left]);
                        item.add(num[right]);

                        list.add(item);
                        left++;
                        right--;
                    } else if (sum < target) {
                        left++;
                    } else {
                        right--;
                    }
                } // while
            } // for:j
        } // for: i

        return list;
    }
}