LeetCode 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.

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]

.

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(1). next(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();
*/