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:
1 public class Vector2D { 2 List<List<Integer>> listOfList; 3 int listIndex; 4 int elemIndex; 5 public Vector2D(List<List<Integer>> 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 */