程序员面试题精选100题(12)－从上往下遍历二元树

题目：输入一颗二元树，从上往下按层打印树的每个结点，同一层中按照从左往右的顺序打印。
例如输入

      8
    /  \
   6    10
  /\     /\
5  7   9  11

输出8   6   10   5   7   9   11。

思路： 使用入队的操作即可

///////////////////////////////////////////////////////////////////////
// Print a binary tree from top level to bottom level
// Input: pTreeRoot - the root of binary tree
///////////////////////////////////////////////////////////////////////
void PrintFromTopToBottom(BTreeNode *pTreeRoot)
{
      if(!pTreeRoot)
            return;

      // get a empty queue
      deque<BTreeNode *> dequeTreeNode;

      // insert the root at the tail of queue
      dequeTreeNode.push_back(pTreeRoot);

      while(dequeTreeNode.size())
      {
            // get a node from the head of queue
            BTreeNode *pNode = dequeTreeNode.front();
            dequeTreeNode.pop_front();

            // print the node
            cout << pNode->m_nValue << ' ';

            // print its left child sub-tree if it has
            if(pNode->m_pLeft)
                  dequeTreeNode.push_back(pNode->m_pLeft);
            // print its right child sub-tree if it has
            if(pNode->m_pRight)
                  dequeTreeNode.push_back(pNode->m_pRight);
      }
}