#include <iostream>

 #include <queue>

 using namespace std;

 struct TreeNode {

     int val;

     TreeNode *left;

     TreeNode *right;

     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 };

 void PrintFromTop2Bottom(TreeNode *root)

 {

     if(root == )

         return ;

     queue<TreeNode* > QueueTreeNode;

     QueueTreeNode.push(root);

     while(!QueueTreeNode.empty())

     {

         TreeNode *temp = QueueTreeNode.front();

         cout<<temp->val<<" ";

         QueueTreeNode.pop();

         if(temp->left != )

             QueueTreeNode.push(temp->left);

         if(temp->right != )

             QueueTreeNode.push(temp->right);

     }

 }

 int main()

 {

     TreeNode *Root = new TreeNode();

     TreeNode *node1 = new TreeNode();

     TreeNode *node2 = new TreeNode();

     TreeNode *node3 = new TreeNode();

     TreeNode *node4 = new TreeNode();

     TreeNode *node5 = new TreeNode();

     TreeNode *node6 = new TreeNode();

     Root->left = node1;

     Root->right = node2;

     node1->left = node3;

     node1->right = node4;

     node2->left = node5;

     node2->right = node6;

     cout<<""<<endl;

     cout<<"  1       2"<<endl;

     cout<<"3   4   5   6"<<endl<<endl;

     cout<<"从上往下打印二叉树：";

     PrintFromTop2Bottom(Root);

 }

从上往下打印二叉树：