原题链接在这里：

题目：

Implement an iterator to flatten a 2d vector.

Example:

Input: 2d vector =[  [1,2],  [3],  [4,5,6]]Output: [1,2,3,4,5,6]Explanation: By calling next repeatedly until hasNext returns false,              the order of elements returned by next should be: [1,2,3,4,5,6].

Follow up:

As an added challenge, try to code it using only  or .

题解：

用两个index 分别记录list 的 index 和当前 list的element index.

Time Complexity: Vector2D() O(1). hasNext() O(vec2d.size()). next() O(1). Space: O(1).

AC Java:

1 public class Vector2D { 2     List
     
      
       > listOfList; 3     int listIndex; 4     int elemIndex; 5     public Vector2D(List
       
        
         > vec2d) { 6         listOfList = vec2d; 7         listIndex = 0; 8         elemIndex = 0; 9     }10 11     public int next() {12         return listOfList.get(listIndex).get(elemIndex++);13     }14 15     public boolean hasNext() {16         while(listIndex < listOfList.size()){17             if(elemIndex < listOfList.get(listIndex).size()){18                 return true;19             }else{20                 listIndex++;21                 elemIndex = 0;22             }23         }24         return false;25     }26 }27 28 /**29  * Your Vector2D object will be instantiated and called as such:30  * Vector2D i = new Vector2D(vec2d);31  * while (i.hasNext()) v[f()] = i.next();32  */
        
       
      
     

Follow up 要用Iterator class.

Time Complexity:  Vector2D() O(1). hasNext() O(vec2d.size()). next() O(1). Space: O(1).

AC Java:

1 public class Vector2D implements Iterator
     
       { 2     Iterator
      
       
        > i; 3     Iterator
        
          j; 4      5     public Vector2D(List
         
          
           > vec2d) { 6 i = vec2d.iterator(); 7 } 8 9 @Override10 public Integer next() {11 return j.next();12 }13 14 @Override15 public boolean hasNext() {16 while((j==null || !j.hasNext()) && i.hasNext()){17 j = i.next().iterator();18 }19 20 return j!=null && j.hasNext();21 }22 }23 24 /**25 * Your Vector2D object will be instantiated and called as such:26 * Vector2D i = new Vector2D(vec2d);27 * while (i.hasNext()) v[f()] = i.next();28 */
          
         
        
       
      
     

类似.