【思路】广度优先遍历，队列实现
1 class Solution 2 { 3 public: 4 vector<int> PrintFromTopToBottom(TreeNode* root) 5 { 6 queue<TreeNode*> Queue; 7 vector<int> res; 8 if(root == NULL) 9 return res; 10 Queue.push(root); 11 while(!Queue.empty()) 12 { 13 res.push_back(Queue.front()->val); 14 if(Queue.front()->left != NULL) 15 Queue.push(Queue.front()->left); 16 if(Queue.front()->right != NULL) 17 Queue.push(Queue.front()->right); 18 Queue.pop(); 19 } 20 return res; 21 } 22 };