找到第M个节点（同时保存下M前的节点），对之后的（N-M+1）个节点依次做反转
然后把反转后的链表头尾和原链表衔接上
反转制定个数的节点代码值得学习：
struct ListNode * prev, *curr, *next;
curr = q;
prev = next = NULL;
int k = n-m+1;
while(k--){
next = curr->next;
curr->next = prev;
prev = curr;
curr = next;
}
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* struct ListNode *next;
* };
*/
struct ListNode* reverseBetween(struct ListNode* head, int m, int n) {
if(head == NULL)
return NULL;
struct ListNode * dummy = calloc(1, sizeof(struct ListNode));
dummy->next = head;
struct ListNode * p , *q;
p = dummy;
q = head;
//m = m -1;
int index = m -1;
while(index--){
p = p->next;
q = q->next;
}
//q pointer to Mth node
//p pointer to M-1 th node
//save this as last node in
//rever m-n+1 nodes after from Mth
struct ListNode * prev, *curr, *next;
curr = q;
prev = next = NULL;
int k = n-m+1;
while(k--){
next = curr->next;
curr->next = prev;
prev = curr;
curr = next;
}
//here prev is the revered linked list head node
//curr is next
q->next = curr;
p->next = prev;
return dummy->next;
}
