# 99. Recover Binary Search Tree

## 刷题内容

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

Two elements of a binary search tree (BST) are swapped by mistake.

Recover the tree without changing its structure.

Example 1:

Input: [1,3,null,null,2]

1
/
3
\
2

Output: [3,1,null,null,2]

3
/
1
\
2
Example 2:

Input: [3,1,4,null,null,2]

3
/ \
1   4
/
2

Output: [2,1,4,null,null,3]

2
/ \
1   4
/
3


˼· **- ʱ�临�Ӷ�: O(n)*- �ռ临�Ӷ�: O(1)***

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/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode* n1;
TreeNode* n2;
TreeNode* pre = new TreeNode(INT_MIN);
void travel(TreeNode* root){
if(!root)
return;
travel(root ->left);
if(!n1 && root ->val <= pre ->val)
n1 = pre;
if(pre && n1 && root ->val <= pre ->val)
n2 = root;
pre = root;
travel(root ->right);
}
void recoverTree(TreeNode* root) {
travel(root);
swap(n1 ->val,n2 ->val);
}
};