Flatten 2D Vector

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].

分析：https://segmentfault.com/a/1190000003791233

用一个数组表示每个List的迭代器，然后再记录一个变量，用来表示当前用到了第几个迭代器。

public class Vector2D {

    List<Iterator<Integer>> its;
    int curr = 0;

    public Vector2D(List<List<Integer>> vec2d) {
        this.its = new ArrayList<Iterator<Integer>>();
        for (List<Integer> l : vec2d) {
            // 只将非空的迭代器加入数组
            if (l.size() > 0) {
                this.its.add(l.iterator());
            }
        }
    }

    public int next() {
        Integer res = its.get(curr).next();
        // 如果该迭代器用完了，换到下一个
        if (!its.get(curr).hasNext()) {
            curr++;
        }
        return res;
    }

    public boolean hasNext() {
        return curr < its.size() && its.get(curr).hasNext();
    }
}