Capable to k-vector input too:

class ZigzagIterator {    int x;        int i;    int max_x;    vector
     
      
       *> l;        void moveon()    {        int oldi= i;        i = (i + 1) % l.size();        x += i <= oldi;    }public:    ZigzagIterator(vector
       
        & v1, vector
        
         & v2)     {        i = x = 0;        max_x = max(v1.size(), v2.size());        if(v1.size() > 0)    l.push_back(&v1);        if(v2.size() > 0)    l.push_back(&v2);    }    int next()     {        int ret = (*l[i])[x];        while(moveon(), x < max_x && l[i]->size() <= x);return ret; 　　　　 return ret;    }    bool hasNext()     {        return x < max_x;    }};