层序遍历，队列

class Solution {
public:
    vector<int> PrintFromTopToBottom(TreeNode* root) {
        queue<TreeNode*> s;
        vector<int> ans;
        if(!root) return ans;
        s.push(root);
        while(!s.empty()){
            //int size=s.size();
            TreeNode* tmp;
            tmp=s.front();
            s.pop();
            ans.push_back(tmp->val);
            if(tmp->left) s.push(tmp->left);
            if(tmp->right) s.push(tmp->right);
        }
        return ans;
    }
};