Flatten 2D Vector -- LeetCode

Implement an iterator to flatten a 2d vector.

For example,
Given 2d vector =

[
  [1,2],
  [3],
  [4,5,6]
]

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 = 0;
    }

    int next() {
        int res = vec[nextRow][nextCol++];
        if (nextCol == vec[nextRow].size()) {
            nextCol = 0;
            nextRow++;
        }
        return res;
    }
    
    //skip empty sub arrays
    bool hasNext() {
        while (nextRow < vec.size() && nextCol == vec[nextRow].size()) {
                nextCol = 0;
                nextRow++;
        }
        return nextRow < vec.size();
    }
};

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