Flatten 2D Vector -- LeetCode

Implement an iterator to flatten a 2d vector.

For example,
Given 2d vector =

[
  [,],
  [],
  [,,]
]

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,2,3,4,5,6].

 class Vector2D {
 public:
     int height, nextRow, nextCol;
     vector<vector<int> > vec;
     Vector2D(vector<vector<int>>& vec2d) {
         vec = vec2d;
         height = vec.size();
         nextRow = nextCol = ;
     }

     int next() {
         int res = vec[nextRow][nextCol++];
         if (nextCol == vec[nextRow].size()) {
             nextCol = ;
             nextRow++;
         }
         return res;
     }

     //skip empty sub arrays
     bool hasNext() {
         while (nextRow < vec.size() && nextCol == vec[nextRow].size()) {
                 nextCol = ;
                 nextRow++;
         }
         return nextRow < vec.size();
     }
 };

 /**
  * Your Vector2D object will be instantiated and called as such:
  * Vector2D i(vec2d);
  * while (i.hasNext()) cout << i.next();
  */