/**

 * 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>> levelOrderBottom(TreeNode* root) {

        result.clear();

        if(!root) return result;

        vector<int> level;

        TreeNode * current;

        queue<TreeNode * > queToPop;

        queue<TreeNode * > queToPush;

        queToPop.push(root);

        while(!queToPop.empty())

        {

            while(!queToPop.empty())

            {

                current = queToPop.front();

                queToPop.pop();

                level.push_back(current->val);

                if(current->left)

                {

                    queToPush.push(current->left); //若使用一个队列，这里应该是queToPop.push(...)，只要queToPop不为空，就继续pop和push

                }

                if(current->right)

                {

                    queToPush.push(current->right);

                }

            }

            result.push_back(level);

            level.clear();

            swap(queToPop, queToPush);

        }

        reverse(result.begin(), result.end()); //逆序vector的方法

        return result;

    }

private:

    vector<vector<int>> result;

};