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)

Discuss

class Solution
{
public:
    vector<vector<int> > fourSum(vector<int> &num, int target)
    {
        vector<vector<int> > Result;

        int Size = num.size();

        if (Size < 4)
        {
            return Result;
        }


        // sort the array
        sort(num.begin(), num.end());

        for (int Index_first = 0; Index_first < (Size - 3); Index_first++)
        {
            int First = num[Index_first];

            if ((Index_first != 0) && (num[Index_first - 1] == num[Index_first]))
            {
                continue;
            }

            for (int Index_second = Index_first + 1; Index_second < (Size - 2); Index_second++)
            {
                if ((Index_second != (Index_first + 1)) && (num[Index_second - 1] == num[Index_second]))
                {
                    continue;
                }

                int Second = num[Index_second];

                int Index_third = Index_second + 1;
                int Index_foud = Size - 1;

                while (Index_third < Index_foud)
                {
                    int Third = num[Index_third];
                    int Fourd = num[Index_foud];

                    int Sum = First + Second + Third + Fourd;

                    if (Sum == target)
                    {
                        vector<int> Tmp;
                        Tmp.push_back(First);
                        Tmp.push_back(Second);
                        Tmp.push_back(Third);
                        Tmp.push_back(Fourd);

                        Result.push_back(Tmp);

                        Index_third++;
                        while ((Index_third <= (Size - 1)) && (num[Index_third] == num[Index_third - 1]))
                        {
                            Index_third++;
                        }

                        Index_foud--;
                        while ((Index_foud > Index_second) && (num[Index_foud] == num[Index_foud + 1]))
                        {
                            Index_foud--;
                        }
                    }

                    if (Sum < target)
                    {
                        Index_third++;

                        while ((Index_third < Size) && (num[Index_third] == num[Index_third - 1]))
                        {
                            Index_third++;
                        }
                    }

                    if (Sum > target)
                    {
                        Index_foud--;

                        while ((Index_foud > Index_second) && (num[Index_foud] == num[Index_foud + 1]))
                        {
                            Index_foud--;
                        }
                    }
                }

            }

        }

        return Result;

    }
};