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 = ;

     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() > ) {
                 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();
     }
 }