Leetcode -- Binary Tree Level Order Traversal II

Here we use recursion as stack

 /**  
  * Definition for binary tree  
  * 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) {  
     vector<vector<int> > result;  
     if(root == NULL) {  
       return result;  
     }  
     vector<TreeNode *> output;  
     output.emplace_back(root);  
     levelOrderBottomHelper(output, result);  
     return result;  
   }  
   void levelOrderBottomHelper(vector<TreeNode *>& output, vector<vector<int> >& result) {  
     vector<TreeNode *> nodes;  
     for(int i = 0; i < output.size(); ++i) {  
       if(output[i]->left != NULL) {  
         nodes.emplace_back(output[i]->left);  
       }  
       if(output[i]->right != NULL) {  
         nodes.emplace_back(output[i]->right);  
       }  
     }  
     if(!nodes.empty()) {  
       levelOrderBottomHelper(nodes, result);  
     }   
     vector<int> temp;  
     for(TreeNode * node : output) {  
       temp.emplace_back(node->val);  
     }  
     result.emplace_back(temp);  
   }  
 };