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)

class Solution 
{
public:
    vector<vector<int> > fourSum(vector<int> &num, int target) 
    {
        int n = num.size();
        vector<vector<int>> res;
        if(n<4)
            return res;
        heap_sort(num);
        for(int i=0; i<n; i++)
        {
            if(i!=0 && num[i] == num[i-1])
                continue;
            for(int j=i+1; j<n; j++)
            {
                if(j!=i+1 && num[j] == num[j-1])
                    continue;
                int low = j+1,high=n-1;
                while(low < high)
                {
                    int sum = num[i] + num[j] + num[low] + num[high];
                    if(sum > target)
                        high--;
                    else if(sum < target)
                        low++;
                    else
                    {
                        vector<int> temp;
                        temp.push_back(num[i]);
                        temp.push_back(num[j]);
                        temp.push_back(num[low]);
                        temp.push_back(num[high]);
                        res.push_back(temp);
                        low++;
                        high--;
                        while(low<high && num[low] == num[low-1])
                            low++;
                        while(low<high && num[high] == num[high+1])
                            high--;
                    }
                }
            }
        }
        return res;
    }
    void swap(vector<int>& num, int i, int j)
    {
        int temp = num[i];
        num[i] = num[j];
        num[j] = temp;
    }
    void sink(vector<int>& num, int s, int end)
    {
        while(s < end)
        {
            int k = 2*s+1;
            if(k>end)
                return;
            if(k<end && num[k+1] > num[k])
                k++;
            if(num[s] > num[k])
                return;
            swap(num,s,k);
            s = k;
        }
    }
    void heap_sort(vector<int>& num)
    {
        int n = num.size()-1;
        int i;
        for(i=(n-1)/2; i>=0; i--)
            sink(num,i,n);
        i = n;
        while(i>0)
        {
            swap(num,0,i);
            i--;
            sink(num,0,i);
        }
    }
};

LeetCode--4Sum