其实本题没有我想的那么难，就是到很普通的基础题，只需要注意在每次hasNext()的时候，set()一下就成了(也许col outOfIndex)

    时间复杂度：

    • Constructor: 251. Flatten 2D Vector - 图1
    • set():
      • 251. Flatten 2D Vector - 图2: inner array的数量
      • 251. Flatten 2D Vector - 图3：总共的number
      • 一共调用251. Flatten 2D Vector - 图4次，真正运行过251. Flatten 2D Vector - 图5次，所以平均时间复杂度是251. Flatten 2D Vector - 图6
    • next()/hasNext(): 既可能是，也可能是

    代码如下：

    1. class Vector2D {
    3.   private int row;
    4.   private int col;
    5.   private int[][] arr;
    7.   public Vector2D(int[][] v) {
    8.     this.arr = v;
    9.     this.row = 0;
    10.     this.col = 0;
    11.   }
    13.   private void set() {
    14.     while (row < arr.length && col == arr[row].length) {
    15.       col = 0;
    16.       ++row;
    17.     }
    18.   }
    20.   public int next() {
    21.     if (!hasNext()) {
    22.       throw new java.util.NoSuchElementException();
    23.     }
    24.     return arr[row][col++];
    25.   }
    27.   public boolean hasNext() {
    28.     set();
    29.     return row < arr.length;
    30.   }
    31. }
    33. /**
    34.  * Your Vector2D object will be instantiated and called as such:
    35.  * Vector2D obj = new Vector2D(v);
    36.  * int param_1 = obj.next();
    37.  * boolean param_2 = obj.hasNext();
    38.  */