从上往下打印二叉树

class Solution {
public:
    queue<TreeNode*>Que;
    vector<int> PrintFromTopToBottom(TreeNode* root) {
        if(root != NULL){
            Que.push(root);
        }
        vector<int> res;
        while(!Que.empty()){
            TreeNode *elem = Que.front();
            Que.pop();
            res.push_back(elem->val);
            if(elem->left != NULL){
                Que.push(elem->left);
            }
            if(elem->right != NULL){
                Que.push(elem->right);
            }
        }
        return res;
    }
};