从上到下按层打印二叉树，每层打印顺序从左到右

 1 class Solution{
 2 public :
 3 vector<int> PrintFromTopToBottom(TreeNode* root){
 4 vector<int> vec;
 5 queue<TreeNode*> Q;
 6 if(root==NULL) return vec;
 7 Q.push(root);
 8 while(!Q.empty())
 9 {
10 TreeNode* p = Q.front();
11 Q.pop();
12 vec.push_back(p->val);
13 if(p->left)
14 Q.push(p->left);
15 if(p->right)
16 Q.push(p->right);
17 }
18 return vec;
19 }
20 };