156. Binary Tree Upside Down

Postorder traversal and then construct the tree from downside to upside.

1:  /**  
2:   * Definition for a binary tree node.  
3:   * struct TreeNode {  
4:   *   int val;  
5:   *   TreeNode *left;  
6:   *   TreeNode *right;  
7:   *   TreeNode(int x) : val(x), left(NULL), right(NULL) {}  
8:   * };  
9:   */  
10:  class Solution {  
11:  public:  
12:    TreeNode* upsideDownBinaryTree(TreeNode* root) {  
13:      if (root == NULL) return root;  
14:      TreeNode *res = root;  
15:      while (res->left) res = res->left;  
16:      helper(root);  
17:      return res;  
18:    }  
19:    TreeNode *helper(TreeNode *root) {  
20:      if (root->left == NULL && root->right == NULL) return root;  
21:      TreeNode *nr = helper(root->left);  
22:      nr->left = root->right;  
23:      nr->right = root;  
24:      root->left = NULL;  
25:      root->right = NULL;  
26:      return root;  
27:    }  
28:  };