251. Flatten 2D Vector, iterator

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].

Solution：Iterator

思路：
Time Complexity: O() Space Complexity: O()

Solution Code：

public class Vector2D implements Iterator<Integer> {

    private Iterator<List<Integer>> i;
    private Iterator<Integer> j;
    
    public Vector2D(List<List<Integer>> vec2d) {
        i = vec2d.iterator();
    }

    @Override
    public Integer next() {
        hasNext();
        return j.next();
    }

    @Override
    public boolean hasNext() {
        while ((j == null || !j.hasNext()) && i.hasNext())
            j = i.next().iterator();
        return j != null && j.hasNext();
    }
}