[剑指Offer] 22.从上往下打印二叉树

【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 };