BST pre-order Traversal (Recursive Code and Complexity)

Pre-condition:

typedef struct intnode {
 int info;
 node *left, *right;
}node;

Algorithm:
step 1: if pointer is null, return;
step 2: visit root
step 3: visit left sub-tree, repeat step 1
step 4: visit right sub-tree, repeat step 1
step 5: stop

Code:

void preorder(node *p){
 // if pointer null, return
 if( p == NULL )
  return; 
  
 // visit root
 printf("%d ", p->info);
 
 // visit left-sub-tree
 preorder( p->left );
 
 // visit right-sub-tree
 preorder( p->right );
}

Complexity:
O(n), because each node has to be visited atleast once and increases linearly for increasing 'n'.