C++AVL树4种旋转详讲(左单旋、右单旋、左右双旋、右左双旋)

template
struct AVLtreeNode
{
    //节点构造函数
	AVLtreeNode(const pair& kv)
		:_left(nullptr)
		,_right(nullptr)
		,_parent(nullptr)
		,_bf(0)
		,_kv(kv)
	{}
    //节点的成员
    //三叉链
	AVLtreeNode* _left;
	AVLtreeNode* _right;
	AVLtreeNode* _parent;
	int _bf;//平衡因子
    //数据使用库里面的pair类存储的kv
	pair _kv;
};
template
class AVLtree
{
	typedef AVLtreeNode Node;
public:
    //构造函数
	AVLtree()
		:_root(nullptr)
	{}
    //四种旋转
	void RotateL(Node* parent)
	void RotateR(Node* parent)
	void RotateLR(Node* parent)
	void RotateRL(Node* parent)
    //插入
	bool Insert(const pair& kv)
    //寻找
	Node* Find(const K& kv)
private:
	Node* _root;
};

bool Insert(const pair& kv)
	{
		if (_root == nullptr)
		{
			_root = new Node(kv);
			return true;
		}
		Node* parent = _root, *cur = _root;
		while (cur)
		{
			//找nulptr，如果已经有这个key了，二叉搜索树的特性不支持冗余，所以返回失败
			if (cur->_kv.first > kv.first)
			{
				parent = cur;
				cur = cur->_left;
			}
			else if (cur->_kv.first _right;
			}
			else
			{
				return false;
			}
		}
		//
		cur = new Node(kv);
		//判断孩子在父亲的左边还是右边
		if (cur->_kv.first > parent->_kv.first)
		{
			parent->_right = cur;
			cur->_parent = parent;
		}
		else
		{
			parent->_left = cur;
			cur->_parent = parent;
		}
		while (parent)
		{
			//影响一条路径所有的祖先
			if (parent->_right == cur)
				parent->_bf++;
			else
				parent->_bf--;
			
			if (parent->_bf == 0)
			{
				//左右平衡了不会再影响祖先了
				break;
			}
			if (parent->_bf == 1 || parent->_bf == -1)
			{
				//当前节点所在子树变了，会影响父亲
				// 继续往上更新
				cur = parent;
				parent = parent->_parent;
			}
			else if (parent->_bf == 2 || parent->_bf == -2)
			{
				//parent所在子树已经不平衡，需要旋转处理一下
				if (parent->_bf == -2)
				{
					if (cur->_bf == -1)
						// 右单旋
						RotateR(parent);
					else // cur->_bf == 1
						RotateLR(parent);
				}
				else // parent->_bf  == 2
				{
					if (cur->_bf == 1)
						// 左单旋
						RotateL(parent);
					else // cur->_bf == -1
						RotateRL(parent);
				}
				break;
			}
			else
			{
				// 插入节点之前，树已经不平衡了，或者bf出错。需要检查其他逻辑
				assert(false);
			}
		}
		return true;
	}

void RotateR(Node* parent)
	{
		//轴点的左，孩子节点
		Node* subL = parent->_left;
		//孩子节点的右
		Node* subLR = subL->_right;
		//我的右当你（轴点）的左
		parent->_left = subLR;
		//调整三叉链
		if (subLR)
			subLR->_parent = parent;
		//你（轴点）做我的右
		subL->_right = parent;
		//调整三叉链
		Node* parentParent = parent->_parent;
		parent->_parent = subL;
 
		if (parent == _root)
		{
			_root = subL;
			_root->_parent = nullptr;
		}
		else
		{
			//轴点的父亲新的孩子节点
			if (parentParent->_left == parent)
				parentParent->_left = subL;
			else
				parentParent->_right = subL;
 
			subL->_parent = parentParent;
		}
 
		subL->_bf = parent->_bf = 0;
	}

void RotateL(Node* parent)
	{
		//轴点的右，孩子节点
		Node* subR = parent->_right;
		//孩子节点的左
		Node* subRL = subR->_left;
		//我的左当你（轴点）的右
		parent->_right = subRL;
		//调整三叉链
		if (subRL)
		{
			subRL->_parent = parent;
		}
		//你（轴点）做我的左
		subR->_left = parent;
		Node* parentparent = parent->_parent;
 
		parent->_parent = subR;
		if (parent == _root)
		{
			if (parentparent->_left == parent)
				parentparent->_left = subR;
			else
				parentparent->_right = subR;
 
			subR->_parent = parentparent;
		}
		else
		{
			subR->_parent = nullptr;
			_root = subR;
		}
 
		subR->_bf = parent->_bf = 0;
 
	}

void RotateLR(Node* parent)
	{
		Node* subL = parent->_left;
		Node* subLR = subL->_right;
		int bf = subLR->_bf;
 
		RotateL(parent->_left);
		RotateR(parent);
 
		// ...平衡因子调节还需要具体分析
		if (bf == -1)
		{
			subL->_bf = 0;
			parent->_bf = 1;
			subLR->_bf = 0;
		}
		else if (bf == 1)
		{
			parent->_bf = 0;
			subL->_bf = -1;
			subLR->_bf = 0;
		}
		else if (bf == 0)
		{
			parent->_bf = 0;
			subL->_bf = 0;
			subLR->_bf = 0;
		}
		else
		{
			assert(false);
		}
	}

void RotateRL(Node* parent)
	{
		Node* subR = parent->_right;
		Node* subRL = subR->_left;
		int bf = subRL->_bf;
 
		RotateR(parent->_right);
		RotateL(parent);
 
		// 平衡因子更新
		if (bf == 1)
		{
			subR->_bf = 0;
			parent->_bf = -1;
			subRL->_bf = 0;
		}
		else if (bf == -1)
		{
			parent->_bf = 0;
			subR->_bf = 1;
			subRL->_bf = 0;
		}
		else if (bf == 0)
		{
			parent->_bf = 0;
			subR->_bf = 0;
			subRL->_bf = 0;
		}
		else
		{
			assert(false);
		}
	}