92. Reverse Linked List II

92. Reverse Linked List II (Medium)

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

Note: 1 ≤ m ≤ n ≤ length of list.

Example:

Input: 1->2->3->4->5->NULL, m = 2, n = 4
Output: 1->4->3->2->5->NULL

Related Topics:
Linked List

Similar Questions:

• Reverse Linked List (Easy)

Solution 1.

// OJ: https://leetcode.com/problems/reverse-linked-list-ii/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        ListNode dummy, *p = &dummy;
        dummy.next = head;
        for (int i = 1; i < m; ++i) p = p->next;
        auto q = p->next, tail = q;
        for (int i = m; i <= n; ++i) {
            auto node = q;
            q = q->next;
            node->next = p->next;
            p->next = node;
        }
        tail->next = q;
        return dummy.next;
    }
};