/**

 * Definition for a binary tree node.

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

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

 * };

 */

/**

最直接的想法是利用层次遍历，然后reverse偶数层的数据

**/

class Solution {

public:

    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {

        if(root==NULL) return {};

        vector<vector<int>> res;

        queue<TreeNode*> q;

        TreeNode* p;

        q.push(root);

        int flag=;

        while(!q.empty()){

            int k=q.size();

            vector<int> level;

            for(int i=;i<k;i++){

                p=q.front();

                q.pop();

                level.push_back(p->val);

                if(p->left) q.push(p->left);

                if(p->right) q.push(p->right);

            }

            if(flag==) reverse(level.begin(),level.end());

            flag=(flag+)%;

            res.push_back(level);

        }

        return res;

    }

};