# 0281. Zigzag Iterator

<https://leetcode.com/problems/zigzag-iterator>

## Description

Given two vectors of integers `v1` and `v2`, implement an iterator to return their elements alternately.

Implement the `ZigzagIterator` class:

* `ZigzagIterator(List<int> v1, List<int> v2)` initializes the object with the two vectors `v1` and `v2`.
* `boolean hasNext()` returns `true` if the iterator still has elements, and `false` otherwise.
* `int next()` returns the current element of the iterator and moves the iterator to the next element.

**Example 1:**

```
**Input:** v1 = [1,2], v2 = [3,4,5,6]
**Output:** [1,3,2,4,5,6]
**Explanation:** By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,3,2,4,5,6].
```

**Example 2:**

```
**Input:** v1 = [1], v2 = []
**Output:** [1]
```

**Example 3:**

```
**Input:** v1 = [], v2 = [1]
**Output:** [1]
```

**Constraints:**

* `0 <= v1.length, v2.length <= 1000`
* `1 <= v1.length + v2.length <= 2000`
* `-231 <= v1[i], v2[i] <= 231 - 1`

**Follow up:** What if you are given `k` vectors? How well can your code be extended to such cases?

**Clarification for the follow-up question:**

The "Zigzag" order is not clearly defined and is ambiguous for `k > 2` cases. If "Zigzag" does not look right to you, replace "Zigzag" with "Cyclic".

**Example:**

```
**Input:** v1 = [1,2,3], v2 = [4,5,6,7], v3 = [8,9]
**Output:** [1,4,8,2,5,9,3,6,7]
```

## ac

```java
public class ZigzagIterator {
    private Queue<int[]> q;
    private List<List<Integer>> lists;

    public ZigzagIterator(List<Integer> v1, List<Integer> v2) {
        lists = new ArrayList<List<Integer>>();
        lists.add(v1);
        lists.add(v2);

        q = new LinkedList<int[]>();
        if (v1.size() > 0) q.offer(new int[] {0, 0});
        if (v2.size() > 0) q.offer(new int[] {1, 0});
    }

    public int next() {
        int[] head = q.poll();
        int res = lists.get(head[0]).get(head[1]);
        if (++head[1] < lists.get(head[0]).size()) q.offer(head);
        return res;
    }

    public boolean hasNext() {
        return !q.isEmpty();
    }
}

/**
 * Your ZigzagIterator object will be instantiated and called as such:
 * ZigzagIterator i = new ZigzagIterator(v1, v2);
 * while (i.hasNext()) v[f()] = i.next();
 */
```