按照上一题使用的层序遍历，只是奇数层和偶数层向level中放入的顺序不一样

• 奇数层 从左到右

• 偶数层 从右到左

其他完全一样

class Solution {
public:
    vector<vector<int>> printFromTopToBottom(TreeNode* root) {
        if(!root)   return {};
        vector<vector<int>> res;
        queue<TreeNode*> q;
        q.push(root);
        bool from_left_to_right = true;

        while(q.size())
        {
            int n = q.size();
            vector<int> level(n);
            for(int i = 0; i < n; ++i)
            {
                auto node = q.front();
                q.pop();
                level[from_left_to_right ? i : n-1-i] = node->val;  // 放入顺序不一样
                if(node->left)  q.push(node->left);
                if(node->right) q.push(node->right);
            }
            res.push_back(level);
            from_left_to_right = !from_left_to_right;
        }
        return res;
    }
};