[LeetCode] 92. Reverse Linked List II

Reverse a linked list from position m to n. Do it in-place and in one-pass.

For example:

Given 1->2->3->4->5->NULL, m = 2 and n = 4,

return 1->4->3->2->5->NULL.

class Solution {
public:
ListNode* reverseBetween(ListNode* head, int m, int n) {
if (head == nullptr || head->next == nullptr || m == n) return head;
if (m != 1) {
ListNode *p = head;
for (m--, n--; m > 1; m--, n--)
p = p->next;
p->next = reverseBetween(p->next, m, n);
return head;
} else {
ListNode *pend = head;
ListNode *prev = nullptr;
ListNode *cur = head;

for (int i = 1; i < n; i++) {
ListNode *tmp = cur->next;
cur->next = prev;
prev = cur;
cur = tmp;
}

ListNode *trail = cur->next;
cur->next = prev;
pend->next = trail;

return cur;
}
}
};