Find nth smallest element in Binary Search Tree - c++

Question

I have written an algorithm for finding nth smallest element in BST but it returns root node instead of the nth smallest one. So if you input nodes in order 7 4 3 13 21 15, this algorithm after call find(root, 0) returns Node with value 7 instead of 3, and for call find(root, 1) it returns 13 instead of 4. Any thoughts ?
Binode* Tree::find(Binode* bn, int n) const
{
if(bn != NULL)
{
find(bn->l, n);
if(n-- == 0)
return bn;
find(bn->r, n);
}
else
return NULL;
}
and definition of Binode
class Binode
{
public:
int n;
Binode* l, *r;
Binode(int x) : n(x), l(NULL), r(NULL) {}
};

Answer 1

It is not possible to efficiently retrieve the n-th smallest element in a binary search tree by itself. However, this does become possible if you keep in each node an integer indicating the number of nodes in its entire subtree. From my generic AVL tree implementation:
static BAVLNode * BAVL_GetAt (const BAVL *o, uint64_t index)
{
if (index >= BAVL_Count(o)) {
return NULL;
}
BAVLNode *c = o->root;
while (1) {
ASSERT(c)
ASSERT(index < c->count)
uint64_t left_count = (c->link[0] ? c->link[0]->count : 0);
if (index == left_count) {
return c;
}
if (index < left_count) {
c = c->link[0];
} else {
c = c->link[1];
index -= left_count + 1;
}
}
}
In the above code, node->link[0] and node->link[1] are the left and right child of node, and node->count is the number of nodes in the entire subtree of node.
The above algorithm has O(logn) time complexity, assuming the tree is balanced. Also, if you keep these counts, another operation becomes possible - given a pointer to a node, it is possible to efficiently determine its index (the inverse of the what you asked for). In the code I linked, this operation is called BAVL_IndexOf().
Be aware that the node counts need to be updated as the tree is changed; this can be done with no (asymptotic) change in time complexity.

Answer 2

There are a few problems with your code:
1) find() returns a value (the correct node, assuming the function is working as intended), but you don't propagate that value up the call chain, so top-level calls don't know about the (possible) found element
.
Binode* elem = NULL;
elem = find(bn->l, n);
if (elem) return elem;
if(n-- == 0)
return bn;
elem = find(bn->r, n);
return elem; // here we don't need to test: we need to return regardless of the result
2) even though you do the decrement of n at the right place, the change does not propagate upward in the call chain. You need to pass the parameter by reference (note the & after int in the function signature), so the change is made on the original value, not on a copy of it
.
Binode* Tree::find(Binode* bn, int& n) const
I have not tested the suggested changes, but they should put you in the right direction for progress