面试题61：按之字形顺序打印二叉树

题目描述
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。

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解：
两个栈

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/*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> > Print(TreeNode* pRoot) {        vector<vector<int>> result;        if(!pRoot)            return result;        stack<TreeNode*> s[2];        int current = 0;        int next = 1;        s[current].push(pRoot);        vector<int> v;        while(!s[current].empty() || ! s[next].empty()){            TreeNode* pCur = s[current].top();            s[current].pop();            v.push_back(pCur->val);            if(current == 0){                if(pCur->left)                    s[next].push(pCur->left);                if(pCur->right)                    s[next].push(pCur->right);            }else{                if(pCur->right)                    s[next].push(pCur->right);                if(pCur->left)                    s[next].push(pCur->left);            }            if(s[current].empty()){                result.push_back(v);                v.clear();                current = 1 - current;                next = 1 - next;            }        }        return result;    }};