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。

1 public class Vector2D {
 2 
 3     List<Iterator<Integer>> its;
 4     int curr = 0;
 5 
 6     public Vector2D(List<List<Integer>> vec2d) {
 7         this.its = new ArrayList<Iterator<Integer>>();
 8         for (List<Integer> l : vec2d) {
 9             // 只将非空的迭代器加入数组
10             if (l.size() > 0) {
11                 this.its.add(l.iterator());
12             }
13         }
14     }
15 
16     public int next() {
17         Integer res = its.get(curr).next();
18         // 如果该迭代器用完了，换到下一个
19         if (!its.get(curr).hasNext()) {
20             curr++;
21         }
22         return res;
23     }
24 
25     public boolean hasNext() {
26         return curr < its.size() && its.get(curr).hasNext();
27     }
28 }