思路：采用队列思想，一边进一边出，先进先出，选择双向队列deque（其实queue已经够了，没必要用deque）
/*
struct TreeNode {
int val;
struct TreeNode *left;
struct TreeNode *right;
TreeNode(int x) :
val(x), left(NULL), right(NULL) {
}
};*/
class Solution {
public:
vector<int> PrintFromTopToBottom(TreeNode* root) {
deque<TreeNode*> temp;
vector<int> res;
if(NULL==root)
return res;
temp.push_back(root);
while(!temp.empty())
{
TreeNode* t=temp.front();
temp.pop_front();
res.push_back(t->val);
if(t->left!=NULL)
temp.push_back(t->left);
if(t->right!=NULL)
temp.push_back(t->right);
}
return res;
}
};
