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.