Find nth smallest element in binary search tree

#A binary search tree was given. Tell the 4th smallest value in it.

// traverse tree in in order, this will give the output in ascending order, take the fourth element or a[3];

int min = -1; // global variable

void traverse(NODE *p) {
    
    static int index = -1;
    
    // for any node, if index is 3, we have found the element do nothing
    if( p == null || index == 3 )
        return;
    
    // else traverse left tree first, i.e. in order traversal
    traverse( p->left );

    // element found somewhere in left sub-tree
    if( index == 3 ) 
        return; 
    
    // element not found yet, use current node info
    index++; min = p->info;

    // traverse right sub-tree
    traverse( p->right );
}

Note: not tested, suggest better methods if possible 

Complexity: O(n)

Number of noded traversed depends upon the depty and the distribution of the binary tree.