【剑指offer 面试题23】从上往下打印二叉树

思路：

　　没啥好说的，BFS。

C++：

 #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);
 }

测试结果：

从上往下打印二叉树：