AVL Tree Implementation Errors

[njames]

Hey I am trying to create a balanced binary tree (AVL tree) but seem to run into a problem when I try to rebalance nodes. My node class looks something like this

struct node
{
    int val, bal, lsize;
    node *left, *right, *parent;
    node(): val(0),bal(0),left(NULL),right(NULL),parent(NULL),lsize(0){}
};

val is the node's value, bal is its balance, and lsize is the size of its left subtree.

Here is my method to check a node's balance and then determine which, if any, rotation is to be performed.

void check(node* &n)
{
	if(n == NULL)
		return;
	else
	{
		if(n->bal > 1)
			left_rotate(n);
		else if(n->bal < -1)
			right_rotate(n);
		check(n->parent);
	}
}

And here is the method to perform a left rotation. The code for a right rotation is pretty much identical.

void left_rotate(node* &root)
{
	node *oldroot=root;
	//perform rotation
	root=root->right;
	oldroot->right=root->left;
	root->left=oldroot;
	root->parent=oldroot->parent;
	oldroot->parent=root;
	oldroot->bal -= (1 + max(root->bal, 0));
	root->bal -= (1 + min(oldroot->bal, 0));
}

For some reason this seems to be causing an infinite loop, can someone please tell me where the error might be.