按之字形顺序打印二叉树

请实现一个函数按照之字形打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右至左的顺序打印，第三行按照从左到右的顺序打印，其他行以此类推。

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

class Solution
{
public:
vector<vector<int> > printBinaryTreeAsZhi( TreeNode* pRoot )
{
if ( pRoot == NULL )
return vector<vector<int> >();

int current = 0;
int next = 1;

vector<vector<int> > result;
vector<int> temp;

stack<TreeNode*> treeStack[2];

treeStack[current].push( pRoot );
while ( !treeStack[0].empty() || !treeStack[1].empty() )
{
TreeNode* pNode = treeStack[current].top();
treeStack[current].pop();

temp.push_back( pNode->val );

if ( current == 0 )
{
if ( pNode->left != NULL )
{
treeStack[next].push( pNode->left );
}
if ( pNode->right != NULL )
{
treeStack[next].push( pNode->right );
}
}
else
{
if ( pNode->right != NULL )
{
treeStack[next].push( pNode->right );
}
if ( pNode->left != NULL )
{
treeStack[next].push( pNode->left );
}
}

if ( treeStack[current].empty() )
{
result.push_back( temp );
temp.clear();
current = 1 - current;
next = 1 - next;
}
}
return result;
}
};