static struct TreeNode* head = NULL;static struct TreeNode* pre = NULL;struct TreeNode* Convert(struct TreeNode* pRootOfTree ) {    // write code here    if (pRootOfTree == NULL)        return NULL;    Convert(pRootOfTree->left);    if (pre == NULL) {        pre = pRootOfTree;        head = pRootOfTree;    }    else {        pre->right = pRootOfTree;        pRootOfTree->left = pre;        pre = pRootOfTree;    }    Convert(pRootOfTree->right);    return head;}