二叉树-----已知先序遍历、中序遍历，重建二叉树

struct BinaryTreeNode
{
	int value;
	BinaryTreeNode *Left;
	BinaryTreeNode *Right;
};

BinaryTreeNode *ConstructCore(int *startPreorder, int *endPreorder, int *startInorder, int *endInorder)
{
//创建一个结节点
	int rootValue = startPreorder[0];
	BinaryTreeNode* root = new BinaryTreeNode();
	root->value = rootValue;
	root->Left = root->Right = nullptr;
	if (startPreorder == endPreorder)
	{
		if (startInorder == endInorder && *startPreorder == *startInorder)
			return root;
		else
			throw std::exception("Invalid input.");
	}
//在中序遍历中找到根结点
	int *rootInorder = startInorder;
	while (startInorder <= endInorder && *rootInorder != rootValue)
		++rootInorder;
	if (rootInorder == endInorder && *rootInorder != rootValue)
		throw std::exception("Invalid Input.");
	int leftLength = rootInorder - startPreorder;
	int* leftPreorderEnd = startPreorder + leftLength;
	if (leftLength > 0)
	{
//构建左子树
		root->Left = ConstructCore(startPreorder + 1, leftPreorderEnd, startInorder, rootInorder - 1);
	}
//构建右子树
	if (leftLength < endPreorder-startPreorder)
	{
		root->Right = ConstructCore(leftPreorderEnd+1,endPreorder,rootInorder+1,endInorder);
	}
	return root;
}

BinaryTreeNode *Construct(int *preorder, int *inorder, int length)
{
	if (preorder == nullptr || inorder == nullptr || length <= 0)
		return nullptr;
	return ConstructCore(preorder, preorder + length - 1, inorder, inorder + length - 1);
}