3 sum allow number used multi times

class Solution
 {
 public:
     vector<vector<int> > ThreeSum(int a[], int n, int target)
     {
         vector<vector<int> > ret;
         for (int i = 0; i < n; i++)
         {
             if(i>0 && a[i] == a[i - 1])
                 continue;
             int begin = i, end = n - 1;
             while (begin <= end)
             {
                 int sum = a[i] + a[begin] + a[end];
                 if (sum == target)
                 {
                     vector<int> res = { a[i], a[begin], a[end] };
                     ret.push_back(res);
                     begin++;
                     end--;
                     while (begin < end && a[begin] == a[begin - 1])
                         begin++;
                     while (begin<end && a[end] == a[end + 1])
                         end--;
                 }
                 else if (sum>target)
                 {
                     end--;
                 }
                 else
                 {
                     begin++;
                 }

             }
         }
         return ret;
     }
};