1、将链表分成前后两个部分：前一个链表长度 >=  后一链表长度；简单2、反转后一个链表；简单3、合并两个链表；简单4、调用以上接口；gameover

typedef struct Node Node_t;
Node_t *list_half(Node_t *head)
{Node_t *rv = NULL;Node_t *start = head, *end = head;while(end && end->next && end->next->next){end = end->next->next;start = start->next;}if(start != NULL){rv = start->next;start->next = NULL;}return rv;
}Node_t *list_reverse(Node_t *head)
{Node_t *rv = head;Node_t *prev = NULL,*current= head,*next = NULL;

while(current){next = current->next;        current->next = prev;prev = current;current = next;  }
rv = prev;return rv;
}/* head_first.len >= head_second.len * or: please check next_second == NULL*/
Node_t *list_merge(Node_t *head_first, Node_t *head_second)
{Node_t *rv = head_first;Node_t *next_first = NULL, *next_second = NULL;#define LEN_DEFENSE
#undef LEN_DEFENSE
#ifdef LEN_DEFENSE Node_t **tail_chek = NULL;

    if(head_first == NULL)         return head_second;
#endifwhile(head_first && head_second){next_first = head_first->next;next_second = head_second->next;head_first->next = head_second;head_second->next = next_first;#ifdef LEN_DEFENSEtail_chek = &head_second->next;
#endifhead_first = next_first;head_second = next_second;}#ifdef LEN_DEFENSE    if(tail_check)    {
    　　*tail_check = head_second;    }
#endifreturn rv;
}Node_t *list_result(Node_t *head)
{Node_t *head_halflist = list_half(head);head_halflist = list_reverse(head_halflist);head = list_merge(head, head_halflist);return head;
}