https://leetcode.com/problems/convert-bst-to-greater-tree/

1. Use reversed inorder traversal:

  1. //32 ms    27.4 MB
  3. /**
  4.  * Definition for a binary tree node.
  5.  * struct TreeNode {
  6.  *     int val;
  7.  *     TreeNode *left;
  8.  *     TreeNode *right;
  9.  *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
  10.  *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
  11.  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
  12.  * };
  13.  */
  14. class Solution {
  15. public:
  16.     TreeNode* convertBST(TreeNode* root) {
  18.         reversed_inorder(root);
  20.         return root;
  21.     }
  23. private:
  24.     void reversed_inorder(TreeNode* root) {
  25.         if(!root)
  26.             return;
  28.         reversed_inorder(root->right);
  29.         root->val = root->val + prev;
  30.         prev = root->val;
  31.         reversed_inorder(root->left);
  32.     }
  34.     int prev = 0;
  35. };

Time Complexity: O(n)
Space Complexity: O(1)