Leetcode: 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.

Note:
Given m, n satisfy the following condition:
1 ≤ m ≤ n ≤ length of list.

分析：头插法。时间复杂度O(n)，空间复杂度O(1)。

class Solution {
public:
    ListNode *reverseBetween(ListNode *head, int m, int n) {
        if(head == NULL || head->next == NULL || m == n) return head;
        
        ListNode *dummy = new ListNode(-1);
        dummy->next = head;
        ListNode *pre_m = dummy;
        
        for(int i = 0; i < m-1; i++)
            pre_m = pre_m->next;
        ListNode *pre = pre_m->next, *cur = pre->next;
        for(int i = m; i < n; i++){
            pre->next = cur->next;
            cur->next = pre_m->next;
            pre_m->next = cur;
            cur = pre->next;
        }
        
        return dummy->next;
    }
};