按之字形顺序打印二叉树

给定一个二叉树，返回该二叉树的之字形层序遍历，（第一层从左向右，下一层从右向左，一直这样交替）

class Solution {
public:
    vector<vector<int> > Print(TreeNode* pRoot) {
        vector<vector<int>> res;
        if(pRoot == NULL)
            return res;
        queue<TreeNode*> que;
        que.push(pRoot);
        bool even = false;
        while(!que.empty()){
            vector<int> vec;
            const int size = que.size();
            for(int i=0; i<size; ++i){
                TreeNode* tmp = que.front();
                que.pop();
                vec.push_back(tmp->val);
                 if(tmp->left != NULL)
                    que.push(tmp->left);
                if(tmp->right != NULL)
                    que.push(tmp->right);
            }
            if(even)
                std::reverse(vec.begin(), vec.end());
            res.push_back(vec);
            even = !even;
        }
        return res;
    }
    
};