PAT Advanced 1066 【Root of AVL Tree】 (25)

分析：AVL树常规操作，靠着模糊的记忆和临时的推导把代码写了个大概，但是由于忘了要用引用将旋转后的结点返回导致没过样例，代码还是比较简洁的，可以作为AVL树的模板了。AVL树其实在理解了BST后就比较好理解和推导了，至于旋转过程自己在纸上也比较简单能推出。

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
struct Tree{
	int data;
	struct Tree *l, *r;
};
int getH(Tree *root){
	if(!root) return 0;
	return max(getH(root->l), getH(root->r))+1;
}

int getBF(Tree *root){ //获取平衡因子Balanced Factor 
	return getH(root->l)-getH(root->r);
}

//左旋 
void LL(Tree* &root){ //需要引用将root返回 
	Tree *tem = root->r;
	root->r = tem->l;
	tem->l = root;
	root = tem;
}
//右旋 
void RR(Tree* &root){
	Tree *tem = root->l;
	root->l = tem->r;
	tem->r = root;
	root = tem;
}
Tree * create(Tree *root, int data){
	if(!root){
		root = new Tree;
		root->data = data;
		root->l = root->r = NULL;
		root->height = 1;
		return root;
	}
	if(data < root->data){
		root->l = create(root->l, data);
		if(getBF(root) == 2){
			if(getBF(root->l) == 1){
				RR(root);
			} else if(getBF(root->l) == -1){
				LL(root->l);
				RR(root);
			}
		}
	} else {
		root->r = create(root->r, data);
		if(getBF(root) == -2){
			if(getBF(root->r) == -1){
				LL(root);
			} else if(getBF(root->r) == 1){
				RR(root->r);
				LL(root);
			}
		}
	}
	return root;
}

int main(){
	//freopen("aa.txt", "r", stdin);
	int n, num;
	cin >> n;
	Tree *root = NULL;
	for(int i = 0; i<n; i++){
		cin >> num;
		root = create(root, num);
	}
	cout << root->data << "\n";
	return 0;
}