Binary Search Trees Switching subtrees - c

Question

So I'm trying to write a function that when given two pointers to nodes in the BST, will 'switch' the subtree locations.
typedef struct NODE {
struct NODE* parent;
struct NODE* left;
struct NODE* right;
}node_t;
This is the node struct I have for the BST.
My function goes along the line of :
void switch_subtree(node_t* a, node_t* b)
{
if (a==NULL || b==NULL)
{
return;
}
if (a->parent->left == a)
{
a->parent->left = b;
}
else
{
a->parent->right = b;
}
if (b->parent->left == b)
{
b->parent->left = a;
}
else
{
b->parent->right = a;
}
nodes * temp = a;
a->parent = b->parent;
b->parent = temp->parent;
}
However, when I run it, it does not properly switch the subtrees.
Can anyone point out any errors Im making and point me in the right direction?
Thanks!!!

Answer 1

Your problem is here:
nodes * temp = a;
a->parent = b->parent;
b->parent = temp->parent;
correctly it should read:
nodes * temp = a->parent;
a->parent = b->parent;
b->parent = temp;
otherwise a->parent is forever lost after line 2.
rationale
wrong approach
The line temp = a will make both pointers temp and a point to the same NODE structure:
+- > +--------+ +- > +--------+
| | | | | |
| | ... | | | ... |
| +--------+ | +--------+
| |
+--------------+ +----------------+
| |
+--------+- > +--------+ | +- > +---------+ |
| | | parent |-+ | | parent |-+
| | | ... | | | ... |
| | +--------+ | +---------+
| | |
+------+ | +---+ | +---+ |
| temp |-+ | a |-+ | b |-+
+------+ +---+ +---+
Changing a->parent in line 2 (a->parent = b->parent) will also change temp->parent as both are just different names for the same component (parent) of the same NODE structure:
+--------+ +---+- > +--------+
| | | | | |
| ... | | | | ... |
+--------+ | | +--------+
| |
| +--------------+
| |
+--------+- > +--------+ | +- > +---------+
| | | parent |---+ | | parent |
| | | ... | | | ... |
| | +--------+ | +---------+
| | |
+------+ | +---+ | +---+ |
| temp |-+ | a |-+ | b |-+
+------+ +---+ +---+
The assignment b->parent = temp->parent doesn't change anything at all, as both b->parent and temp->parent are already pointing at the same node.
- mistake !
alternative
Taking a look at the proposed alternative, temp = a->parent will leave you with the situation sketched below:
+---------+- > +--------+ +- > +--------+
| | | | | | |
| | | ... | | | ... |
| | +--------+ | +--------+
| | |
| +--------------+ +----------------+
| | |
| +- > +--------+ | +- > +---------+ |
| | | parent |-+ | | parent |-+
| | | ... | | | ... |
| | +--------+ | +---------+
| | |
+------+ | +---+ | +---+ |
| temp |-+ | a |-+ | b |-+
+------+ +---+ +---+
After a->parent = b->parent temp is still pointing to the original parent node of the node pointed to by a:
+----------- > +--------+ +- > +--------+
| | | | | |
| | ... | | | ... |
| +--------+ | +--------+
| |
| +-----+----------------+
| | |
| +- > +--------+ | +- > +---------+ |
| | | parent |-+ | | parent |-+
| | | ... | | | ... |
| | +--------+ | +---------+
| | |
+------+ | +---+ | +---+ |
| temp |-+ | a |-+ | b |-+
+------+ +---+ +---+
Finally assigning b->parent = temp will give the node pointed to by b the right parent:
+--------+-- > +--------+ +----- > +--------+
| | | | | | |
| | | ... | | | ... |
| | +--------+ | +--------+
| | |
| +-----------------|--------------------+
| | |
| +- > +--------+ | +- > +---------+ |
| | | parent |---+ | | parent |-+
| | | ... | | | ... |
| | +--------+ | +---------+
| | |
+------+ | +---+ | +---+ |
| temp |-+ | a |-+ | b |-+
+------+ +---+ +---+