要理解这段代码必须把单旋转和双旋转的算法搞明白。其次，要真正理解递归的用法。(注：在gcc环境下编译运行ok)

/*
 * avl tree.
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct AvlNode;
typedef struct AvlNode *Position;
typedef struct AvlNode *AvlTree;

struct AvlNode
{
	int Element;
	AvlTree Left;
	AvlTree Right;
	int Height;
};

int Max(int a, int b) 
{
	return (a >  b) ? a : b ;	
}

static int HeightAvl(Position P)
{
	if (P == NULL)
	{
		return -1;
	}
	else
	{
		return P->Height;
	}

}


static Position SingleRotateWithLeft(Position K2)
{
		Position K1;

		K1 = K2->Left;
		K2->Left = K1->Right;
		K1->Right = K2;

		K2->Height = Max(HeightAvl(K2->Left), HeightAvl(K2->Right)) +1;
		K1->Height = Max(HeightAvl(K1->Left), HeightAvl(K2->Right)) +1;

		return K1;
}

static Position SingleRotateWithRight(Position K2)
{
	Position K1;

	K1 = K2->Right;
	K2->Right = K1->Left;
	K1->Left = K2;

	K2->Height = Max(HeightAvl(K2->Left), HeightAvl(K2->Right)) +1;
	K1->Height = Max(HeightAvl(K1->Left), HeightAvl(K2->Right)) +1;

	return K1;
}

static Position DoubleRotateWithLeft(Position K3)
{
	K3->Left = SingleRotateWithRight(K3->Left);
	return SingleRotateWithLeft(K3);
}

static Position DoubleRotateWithRight(Position K3)
{
	K3->Left = SingleRotateWithLeft(K3->Left);
	return SingleRotateWithRight(K3);
}

AvlTree InsertAvl(int X, AvlTree T)
{
	if (T == NULL)
	{
		T = (AvlTree)malloc(sizeof(AvlTree));
		if (T == NULL)
		{
			printf("out of space!!!\n");
		}
		else
		{
			T->Left = NULL;
			T->Right = NULL;
		}
		T->Element = X;
		T->Height = 0;
	}
	else if (X < T->Element)
	{
		T->Left = InsertAvl(X, T->Left);
		 if (HeightAvl(T->Left) - HeightAvl(T->Right) == 2)
		 {
		 	if (X < T->Left->Element)
		 	{
				T = SingleRotateWithLeft(T);
			}
			else
			{
				T = DoubleRotateWithLeft(T);
			}
		 }
	}
	else if (X > T->Element)
	{
		T->Right = InsertAvl(X, T->Right);
		if (HeightAvl(T->Right) - HeightAvl(T->Left) == 2)
		{
			if (X > T->Right->Element)
			{
				T = SingleRotateWithRight(T);
			}
			else
			{
				T = DoubleRotateWithRight(T);
			}
		}
	}
	T->Height = Max(HeightAvl(T->Left), HeightAvl(T->Right)) + 1;
	return T;
}


int main(void)
{
	AvlTree Tree = NULL;
	InsertAvl(1, Tree);
}