Two nodes of a BST are swapped, correct the BST

PROBLEM :

Two of the nodes of a Binary Search Tree (BST) are swapped. Fix (or correct) the BST.

Input Tree:
         10
        /  \
       5    8
      / \
     2   20

In the above tree, nodes 20 and 8 must be swapped to fix the tree.
Following is the output tree
         10
        /  \
       5    20
      / \
     2   8

--------------------------------------------------------------------------------
SIMPLE c++ IMPLEMENTATION :
--------------------------------------------------------------------------------

/*
typedef struct BST
{
int info ;
struct BST *left ;
struct BST *right ;
}tree ;               */

void swap_node_tree(tree **,tree **) ;
void Two_nodes_swap_tree(tree *,tree **,tree **,tree **,tree **) ;

void treeswap2elemets(tree *root)
{
tree *prev,*first,*middle,*last ;
prev=NULL ;
first=NULL ;
last=NULL ;

Two_nodes_swap_tree(root,&prev,&first,&middle,&last) ;
if(last==NULL)
swap_node_tree(&first,&middle) ;
else
swap_node_tree(&first,&last) ;
}

void Two_nodes_swap_tree(tree *root,tree **prev,tree **first,tree **middle,tree **last)
{
if(root==NULL)
return ;

Two_nodes_swap_tree(root->left,&(*prev),&(*first),&(*middle),&(*last)) ;
if((*prev)!=NULL)
{
if(root->info<(*prev)->info)
{
if((*first)==NULL)
{
(*first)=(*prev) ;
(*middle)=root ;
}
else
(*last)=root ;
}
}
(*prev)=root ;
Two_nodes_swap_tree(root->right,&(*prev),&(*first),&(*middle),&(*last)) ;
}

void swap_node_tree(tree **first,tree **second)
{
int temp ;

temp=(*first)->info ;
(*first)->info=(*second)->info ;
(*second)->info=temp ;
}


---------------------------------------------------------------------------------

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