92. Reverse Linked List II

• Difficulty: Medium

• Topics: Linked List

• Similar Questions:

  • Reverse Linked List

Problem:

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

Solutions:

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        if (head == nullptr)    return nullptr;

        ListNode* dummy = new ListNode(0);
        dummy->next = head;
        ListNode* cur = dummy;
        int index = 0;

        while (cur && index < m - 1) {
            cur = cur->next;
            ++index;
        }

        ListNode* tail = cur;
        cur = cur->next;
        ListNode* reverseLast = cur;
        tail->next = nullptr;

        for (int i = 0; i < n - m + 1; ++i) {
            ListNode* next = cur->next;
            cur->next = tail->next;
            tail->next = cur;
            cur = next;
        }

        reverseLast->next = cur;

        return dummy->next;

    }
};