leetcode Recover Binary Search Tree

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode *p,*q;
TreeNode *prev;
void recoverTree(TreeNode *root)
{
p=q=prev=NULL;
inorder(root);
swap(p->val,q->val);
}
void inorder(TreeNode *root)
{
if(root->left)inorder(root->left);
if(prev!=NULL&&(prev->val>root->val))
{
if(p==NULL)p=prev;
q=root;
}
prev=root;
if(root->right)inorder(root->right);
}
};