把二叉树打印成多行

class Solution {public:        vector<vector<int> > Print(TreeNode* pRoot) {            vector<vector<int> > vec;            if(pRoot == NULL) return vec;            queue<TreeNode*> q;            q.push(pRoot);            while(!q.empty())            {                int lo = 0, hi = q.size();                vector<int> c;                while(lo++ < hi)                {                    TreeNode *t = q.front();                    q.pop();                    c.push_back(t->val);                    if(t->left) q.push(t->left);                    if(t->right) q.push(t->right);                }                vec.push_back(c);            }            return vec;        }};