Given a binary tree, return the bottom-up level order traversal of its nodes' values.

(ie, from left to right, level by level from leaf to root).

For example:

Given binary tree [3,9,20,null,null,15,7],

3

/ \

9 20

/ \

15 7

return its bottom-up level order traversal as:

[

[15,7],

[9,20],

[3]

]

思路：

这个跟 其实差不多的。

唯一不同的是在每一层遍历完之后，得到本层的数值，插入到最终result数组里的时候从头插入，即可实现倒序。

vector
     
      
       > levelOrderBottom(TreeNode* root)    {        queue
       
        que;        vector
        
         
          >result;        if (root == NULL)return result;        que.push(root);        while (!que.empty())        {            int size = que.size();            vector
          
            temp; for (int i = 0; i < size; i++) { TreeNode* node = que.front(); que.pop(); temp.push_back(node->val); if (node->left != NULL)que.push(node->left); if (node->right != NULL)que.push(node->right); } result.insert(result.begin(), temp);//从头部插入即可实现倒序 } return result; }