按之字形顺序打印二叉树

题目：

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。

分析：
使用两个栈。

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;
    }
};