Given two 1d vectors, implement an iterator to return their elements alternately.

For example, given two 1d vectors:

v1 = [1, 2]
v2 = [3, 4, 5, 6]

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be:
[1, 3, 2, 4, 5, 6].

Follow up: What if you are given k 1d vectors? How well can your code be extended to such cases?

Clarification for the follow up question - Update (2015-09-18):
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". For example, given the following input:

[1,2,3]
[4,5,6,7]
[8,9]

It should return [1,4,8,2,5,9,3,6,7].

一刷
题解: 用一个queue装两个list的iterator

public class ZigzagIterator {
    Queue<Iterator<Integer>> list;
    public ZigzagIterator(List<Integer> v1, List<Integer> v2) {
        list = new LinkedList<>();
        if(!v1.isEmpty()) list.add(v1.iterator());
        if(!v2.isEmpty()) list.add(v2.iterator());
    }

    public int next() {
        Iterator poll = list.poll();
        int res = (Integer)poll.next();
        if(poll.hasNext()) list.add(poll);
        return res;
    }

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

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

二刷
怎么比queue快一倍。用flag表示是从第一个还是第二个取

public class ZigzagIterator {
    int v1Index;
    int v2Index;
    List<Integer> v1;
    List<Integer> v2;
    boolean flag;
    public ZigzagIterator(List<Integer> v1, List<Integer> v2) {
        this.v1 = v1;
        this.v2 = v2;
        v1Index = 0;
        v2Index = 0;
        flag = true;
    }

    public int next() {
        if (flag) {
            flag = false;
            return v1.get(v1Index++);
        } else {
            flag = true;
            return v2.get(v2Index++);
        }
    }

    public boolean hasNext() {
        if (v1Index >= v1.size() && v2Index >= v2.size()) {
            return false;
        }
        if (flag) {
            if (v1Index < v1.size()) {
                return true;
            } else {
                flag = false;
                return hasNext();
            }
        } else {
            if (v2Index < v2.size()) {
                return true;
            } else {
                flag = true;
                return hasNext();
            }
        }
    }
}

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