lintcode-二叉树的层次遍历II-70

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]
]

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
   
public:
    
    vector> levelOrderBottom(TreeNode *root) {
        vector > ret;
        if(!root)
            return ret;
        queue que;
        que.push(root);
        
        while(!que.empty()){
            int len=que.size();
            vector base;
            while(len--){
                TreeNode *p=que.front();
                que.pop();
                base.push_back(p->val);
                if(p->left)
                    que.push(p->left);
                if(p->right)
                    que.push(p->right);
            }
            ret.push_back(base);
        }
        reverse(ret.begin(),ret.end());
        return ret;
    }
};