/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> printFromTopToBottom(TreeNode* root) {
        vector<vector<int>> res;
        if(!root) return res;
        queue<TreeNode*> q;
        q.push(root); int i = 1;
        while(q.size())
        {
            int len = q.size();
            vector<int> lev;
            for(int j = 0; j < len; j++)    //队列长度，为该层的节点个数
            {
                auto t = q.front();
                q.pop();
                lev.push_back(t->val);
                // if(i % 2 == 0)    //  偶数行,右到左打印，他的下一层先放左边 这样有错误！！
                // {
                //     if(t->left) q.push(t->left);
                //     if(t->right) q.push(t->right);
                // }
                // else
                // {
                //     if(t->right) q.push(t->right);
                //     if(t->left) q.push(t->left);                    
                // }
                if(t->left) q.push(t->left);
                if(t->right) q.push(t->right);                      
            }
            if(i % 2 == 0) reverse(lev.begin(),lev.end());    //偶数行直接数组倒排就是了！！！
            res.push_back(lev);  i++;
        }
        return res;
    }
};