给出一棵二叉树，返回其节点值从底向上的层次序遍历（按从叶节点所在层到根节点所在的层遍历，然后逐层从左往右遍历）
样例
给出一棵二叉树 {3,9,20,#,#,15,7},
3
/
9 20
/
15 7
按照从下往上的层次遍历为：
[
[15,7],
[9,20],
[3]
]
题目链接：http://www.lintcode.com/zh-cn/problem/binary-tree-level-order-traversal-ii/
跟上一道层次遍历的题目是相同的，为了倒过来访问，只需维护一个栈，把每层的结果亚茹栈中即可。

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
 
 
class Solution {
    /**
     * @param root : The root of binary tree.
     * @return : buttom-up level order a list of lists of integer
     */
public:
    vector<vector<int>> levelOrderBottom(TreeNode *root) {
        // write your code here
        vector<vector<int> > result;
        if (!root) return result;
        queue<TreeNode *> q;
        stack<vector<int> > s;
        q.push(root);
        int len;
        while (!q.empty()) {
            vector<int> res;
            len = q.size();
            while (len--) {
               TreeNode *temp = q.front(); 
               res.push_back(temp->val);
               q.pop();
               if (temp->left) q.push(temp->left);
               if (temp->right)q.push(temp->right);
            }
            s.push(res);
        }
        while (!s.empty()) {
            result.push_back(s.top());
            s.pop();
        }
        return result;
    }
};