LeetCode--Recover Binary Search Tree

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

Recover the tree without changing its structure.

Note:
A solution using O(n) space is pretty straight forward. Could you devise a constant space solution?

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    void recoverTree(TreeNode *root) {   if(root == NULL)return;        vector<TreeNode*> res;pre_search(root,res);if(res.size() == 1)return;bool flag = false;vector<int> temp;for(int i=1; i<res.size(); i++){if(res[i]->val <= res[i-1]->val){if(flag == false){temp.push_back(i-1);flag = true;}else{temp.push_back(i);flag = false;}}}int n = temp.size();if(n == 1){int pre = temp[0];int t = res[pre]->val;res[pre]->val = res[pre+1]->val;res[pre+1]->val = t;}else{int pre = temp[0];int end = temp[1];int t = res[pre]->val;res[pre]->val = res[end]->val;res[end]->val = t;}return ;    }void pre_search(TreeNode* root, vector<TreeNode*>& res){if(root == NULL)return;pre_search(root->left,res);res.push_back(root);pre_search(root->right,res);}};