Binary Tree Level Order Traversal II [LeetCode]

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,#,#,15,7},

    3

   / \

  9  20

    /  \

   15   7

 

return its bottom-up level order traversal as:

[

  [15,7]

  [9,20],

  [3],

]

BFS solution:

 1     vector<vector<int> > levelOrderBottom(TreeNode *root) {

 2         vector<vector<int> > ret;

 3         if(root == NULL)

 4             return ret;

 5         vector<TreeNode *> level;

 6         level.push_back(root);

 7         while(true) {

 8             if(level.size() == 0)

 9                 break;

10             vector<int> nums;

11             vector<TreeNode *> tmp;  

12             for(auto item : level) {

13                 nums.push_back(item->val);

14                 if(item->left != NULL)

15                     tmp.push_back(item->left);

16                 if(item->right != NULL)

17                     tmp.push_back(item->right);

18             }

19             ret.insert(ret.begin(), nums);

20             level = tmp;

21         }

22         return ret;

23     }

DFS solution:

 1     void getLevelNums(TreeNode *root, vector<vector<int> > &ret, int level) {

 2         if(ret.size() < level + 1){

 3             vector<int> nums; 

 4             nums.push_back(root->val);

 5             ret.insert(ret.begin(), nums);

 6         }else if (ret.size() >= level + 1) {

 7             ret[ret.size() - level - 1].push_back(root->val);

 8         }

 9 

10         if(root->left != NULL)

11             getLevelNums(root->left, ret, level + 1);

12 

13         if(root->right != NULL)

14             getLevelNums(root->right, ret, level + 1);

15         

16     }

17     vector<vector<int> > levelOrderBottom(TreeNode *root) {

18         vector<vector<int> > ret;

19         if(root == NULL)

20             return ret;

21         getLevelNums(root, ret, 0);

22         return ret;

23     }