把二叉树打印成多行

从上到下按层打印二叉树，同一层结点从左至右输出。每一层输出一行。

C++:

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/
class Solution {
public:
        vector<vector<int> > Print(TreeNode* pRoot) {
            vector<vector<int> > res ;
            if (pRoot == NULL)
                return res ;
            queue<TreeNode*> q ;
            q.push(pRoot) ;
            while(!q.empty()){
                vector<int> list ;
                int len = q.size() ;
                for(int i = 0 ; i < len ; i++){
                    TreeNode* node = q.front() ;
                    q.pop() ;
                    list.push_back(node->val) ;
                    if (node->left != NULL)
                        q.push(node->left) ;
                    if (node->right != NULL)
                        q.push(node->right) ;
                }
                if (list.size() != 0)
                   res.push_back(list) ;
            }
            return res ;
        }
    
};