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?

/**

 * Definition for binary tree

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

class Solution {

private:

    void swap(int& a,int&b)

    {

        int tmp=a;

        a=b;

        b=tmp;

    }

    TreeNode* findlarge(TreeNode* root,int val)

    {

        if(root==NULL) return NULL;

        TreeNode* result=NULL;

        if(root->val>val) 

        {

            val=root->val;

            result=root;

        }

        TreeNode* p1=findlarge(root->left,val);

        TreeNode* p2=findlarge(root->right,val);

        if(p1!=NULL) result=p1;

        if(p2!=NULL)

        {

            if(result==NULL) result=p2;

            else 

                if(result->val<p2->val) result=p2;

        }

        return result;

    }    

    TreeNode* findsmall(TreeNode* root,int val)

    {

        if(root==NULL) return NULL;

        TreeNode* result=NULL;

        if(root->val<val) 

        {

            val=root->val;

            result=root;

        }

        TreeNode* p1=findsmall(root->left,val);

        TreeNode* p2=findsmall(root->right,val);

        if(p1!=NULL) result=p1;

        if(p2!=NULL)

        {

            if(result==NULL) result=p2;

            else 

                if(result->val>p2->val) result=p2;

        }

        return result;

    }

public:

    void recoverTree(TreeNode *root) 

    {

        if(root==NULL) return;

        TreeNode* left=findlarge(root->left,root->val);

        TreeNode* right=findsmall(root->right,root->val);

        if(left!=NULL && right!=NULL)

        {        

            swap(left->val,right->val);

            return;

        }

        if(right!=NULL)

        {

            swap(root->val,right->val);

            return;

        }

        if(left!=NULL)

        {

            swap(root->val,left->val);

            return;

        }

        recoverTree(root->left);

        recoverTree(root->right);

    }

};