/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* left;
    Node* right;
    Node* next;

    Node() : val(0), left(NULL), right(NULL), next(NULL) {}

    Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}

    Node(int _val, Node* _left, Node* _right, Node* _next)
        : val(_val), left(_left), right(_right), next(_next) {}
};
*/

class Solution {
public:
    Node* connect(Node* root) {
        if(!root){
            return NULL;
        }
        queue<Node*> now,next;
        now.push(root);
        while(!now.empty()){
            Node* pre=NULL;
            while(!now.empty()){
                auto p=now.front();
                if(pre){
                    pre->next=p;
                }
                pre=p;
                if(p->left){
                    next.push(p->left);
                }
                if(p->right){
                    next.push(p->right);
                }
                now.pop();
            }
            pre->next=NULL;
            if(!next.empty()){
                swap(now,next);
            }
        }
        return root;
    }
};