/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        //定义一个队列
        queue<TreeNode*>q;
        //定义一个二维数组用于返回结果
        vector<vector<int> >ans;
        //先将根节点入队
        if(root)
            q.push(root);
        while(!q.empty())
        {
            //定义一个一维数组用于存放每一层节点的值
            vector<int>temp;
            int n = q.size();//队列的长度
            for(int i = 0; i < n; i++)
            {
                //访问队首元素
                TreeNode* t = q.front();
                //队首元素出队
                q.pop();
                //将队首元素的值存放到一维数组中
                temp.push_back(t->val);
                //访问左子树
                if(t->left)
                    q.push(t->left);
                //访问右子树
                if(t->right)
                    q.push(t->right);
            }
            ans.push_back(temp);
        }
        reverse(ans.begin(), ans.end());//反转二维数组中的结果
        return ans;
    }
};