重建二义树

BinaryTreeNode* Construct(int* preorder, int* inorder, int length)

{

    if(preorder == NULL || inorder == NULL || length <= 0)    

        return NULL;

    return ConstructCore(preorder, preorder + length - 1,

        inorder, inorder + length - 1);

}

BinaryTreeNode* ConstructCore

(

    int* startPreorder, int* endPreorder,

    int* startInorder, int* endInorder

)

{

    // 前序遍历序列的第一个数字是根结点的值

    int rootValue = startPreorder[0];

    BinaryTreeNode* root = new BinaryTreeNode();

    root->m_nValue = rootValue;

    root->m_pLeft = root->m_pRight = NULL;

    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的值

    int* rootInorder = startInorder;

    while(rootInorder <= endInorder && *rootInorder != rootValue)

        ++ rootInorder;

    if(rootInorder == endInorder && *rootInorder != rootValue)

        throw std::exception("Invalid input.");

    int leftLength = rootInorder - startInorder;

    int* leftPreorderEnd = startPreorder + leftLength;

    if(leftLength > 0)

    {

        // 构建左子树

        root->m_pLeft = ConstructCore(startPreorder + 1, leftPreorderEnd,

            startInorder, rootInorder - 1);

    }

    if(leftLength < endPreorder - startPreorder)

    {

        // 构建右子树

        root->m_pRight = ConstructCore(leftPreorderEnd + 1, endPreorder,

            rootInorder + 1, endInorder);

    }

    return root;

}