递归解法：
（1）如果二叉树为空，返回真
（2）如果二叉树不为空，如果左子树和右子树都是AVL树并且左子树和右子树高度相差不大于1，返回真，其他返回假

参考代码：
/********************************************************* - Copyright (C): 2016 - File name : isavl.cpp - Author : - zxn - - Date : 2016年08月08日 星期一 12时53分51秒 - Description : * *******************************************************/
#include <iostream>
#include <cstdlib>

#define MAX 4
using namespace std;

struct tree
{
    int data;
    struct tree *left;
    struct tree *right;
};

typedef struct tree treenode;
typedef treenode *btree;


//插入结点
btree insertnode(btree root, int value)
{
    btree newnode;
    btree temp = root;
    //记录父节点
    btree back;
    //创建新的结点
    newnode = new treenode;
    newnode->data = value;
    newnode->left = NULL;
    newnode->right = NULL;

    if (root == NULL)
    {
        return newnode;
    }
    else
    {
        while (temp != NULL)
        {

            back = temp;
            if (temp->data > value)
            {
                temp = temp->left;
            }
            else
            {
                temp = temp->right;
            }
        }
        if (back->data > value)
        {
            back->left = newnode;
        }
        else
        {
            back->right = newnode;
        }
    }
    return root;
}

//二叉查找树的中序遍历
void Printtree(btree root)
{
    if (root)
    {
        Printtree(root->left);
        cout << root->data << " ";
        Printtree(root->right);
    }
}

//判断这棵树是否是平衡二叉树
bool isAVL(btree root, int &height)
{
    if (root == NULL)
    {
        //空树
        height = 0;
        return true;
    }
    int heightLeft;
    bool resultLeft = isAVL(root->left, heightLeft);
    int heightRight;
    bool resultRight = isAVL(root->right, heightRight);

    //左子树和右子树都是AVL树,并且高度相差不大于1.返回真
    if (resultLeft && resultRight && abs(heightLeft - heightRight) <= 1)
    {
        height = max(heightLeft, heightRight) + 1;
        return true;
    }
    else
    {
        height = max(heightLeft, heightRight) + 1;
        return false;
    }
}

int main()
{
    int height = 0;
    btree root = NULL;

    int data[MAX] = {5,4,6,3};
    //循环插入，形成二叉排序树
    for (int i = 0; i < MAX; i++)
    {
        root = insertnode(root, data[i]);
    }
    //中序遍历
    Printtree(root);
    cout << endl;
    //如果该树是平衡二叉树则输出Yes
    if (isAVL(root, height))
    {
        cout << "Yes" << endl;
    }
    else
    {
        cout << "NO" << endl;
    }

    return  0;
}