以下代码可以正确地按层换行输出二叉树的节点值

void printByLevel(TreeNode* root){
    if(!root) return;
    queue<TreeNode*> q;
    q.push(root);
    while(!q.empty()){
        for(int i= 0; i< q.size();++i){
            TreeNode* cur= q.front();
            q.pop();
            cout<< cur->val<<" ";
            if(cur->left) q.push(cur->left);
            if(cur->right) q.push(cur->right);
        }
        cout<< endl;
    }
}