Flatten 2D vector

Flatten2DVector.java

import java.util.*;

/*
https://leetcode.com/problems/flatten-2d-vector/description/

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].
*/

public class Flatten2DVector implements Iterator<Integer> {

  private List<List<Integer>> vector;
  private int row, col;

  public Flatten2DVector(List<List<Integer>> vec2d) {
    vector = vec2d;
    row = 0;
    col = 0;
  }

  @Override
  public Integer next() {
    if (hasNext()) {
      Integer curr = vector.get(row).get(col);
      col++;
      return curr;
    }
    return null;
  }

  @Override
  public boolean hasNext() {
    while (row < vector.size()) {
      if (col == vector.get(row).size()) {
        col = 0;
        row++;
        while (row < vector.size() && col < vector.get(row).size()
            && vector.get(row).get(col) == null) {
          col++;
        }
      } else {
        return true;
      }
    }
    return false;
  }
}