面试题19：二叉树镜像

递归方法非常简单：

 1 void MirrorRecursively(BinaryTreeNode *pNode)
 2 {
 3     if((pNode == NULL) || (pNode->m_pLeft == NULL && pNode->m_pRight))
 4         return;
 5 
 6     BinaryTreeNode *pTemp = pNode->m_pLeft;
 7     pNode->m_pLeft = pNode->m_pRight;
 8     pNode->m_pRight = pTemp;
 9     
10     if(pNode->m_pLeft)
11         MirrorRecursively(pNode->m_pLeft);  
12 
13     if(pNode->m_pRight)
14         MirrorRecursively(pNode->m_pRight); 
15 }

循环需要利用stack：

 1 void MirrorIteratively(BinaryTreeNode* pRoot)
 2 {
 3     if(pRoot == NULL)
 4         return;
 5 
 6     std::stack<BinaryTreeNode*> stackTreeNode;
 7     stackTreeNode.push(pRoot);
 8 
 9     while(stackTreeNode.size() > 0)
10     {
11         BinaryTreeNode *pNode = stackTreeNode.top();
12         stackTreeNode.pop();
13 
14         BinaryTreeNode *pTemp = pNode->m_pLeft;
15         pNode->m_pLeft = pNode->m_pRight;
16         pNode->m_pRight = pTemp;
17 
18         if(pNode->m_pLeft)
19             stackTreeNode.push(pNode->m_pLeft);
20 
21         if(pNode->m_pRight)
22             stackTreeNode.push(pNode->m_pRight);
23     }
24 }