https://leetcode.com/problems/recover-binary-search-tree/

1. Use inorder traversal:

  1. //32 ms    53.3 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.     void recoverTree(TreeNode* root) {
  17.         inorder(root);
  18.         swap(first->val, second->val);
  19.     }
  21.     void inorder(TreeNode* root) {
  22.         if(!root) return;
  23.         inorder(root->left);
  24.         if(prev && prev->val > root->val){
  25.             if(!first) first = prev;
  26.             second = root;
  27.         }
  28.         prev = root;
  29.         inorder(root->right);
  30.     }
  32. private:
  33.     TreeNode* first;
  34.     TreeNode* second;
  35.     TreeNode* prev;
  36. };

Time complexity: O(n)
Space complexity: O(h)