LeetCode 251. Flatten 2D Vector

原题链接在这里：https://leetcode.com/problems/flatten-2d-vector/

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 iterators in C++ or iterators in Java.

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:

 public class Vector2D {
     List<List<Integer>> listOfList;
     int listIndex;
     int elemIndex;
     public Vector2D(List<List<Integer>> vec2d) {
         listOfList = vec2d;
         listIndex = 0;
         elemIndex = 0;
     }

     public int next() {
         return listOfList.get(listIndex).get(elemIndex++);
     }

     public boolean hasNext() {
         while(listIndex < listOfList.size()){
             if(elemIndex < listOfList.get(listIndex).size()){
                 return true;
             }else{
                 listIndex++;
                 elemIndex = 0;
             }
         }
         return false;
     }
 }

 /**
  * Your Vector2D object will be instantiated and called as such:
  * Vector2D i = new Vector2D(vec2d);
  * while (i.hasNext()) v[f()] = i.next();
  */

Follow up 要用Iterator class.

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

AC Java:

 public class Vector2D implements Iterator<Integer> {
     Iterator<List<Integer>> i;
     Iterator<Integer> j;

     public Vector2D(List<List<Integer>> vec2d) {
         i = vec2d.iterator();
     }

     @Override
     public Integer next() {
         return j.next();
     }

     @Override
     public boolean hasNext() {
         while((j==null || !j.hasNext()) && i.hasNext()){
             j = i.next().iterator();
         }

         return j!=null && j.hasNext();
     }
 }

 /**
  * Your Vector2D object will be instantiated and called as such:
  * Vector2D i = new Vector2D(vec2d);
  * while (i.hasNext()) v[f()] = i.next();
  */

类似Flatten Nested List Iterator.