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Reverse Linked List II

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Description

https://leetcode.com/problems/reverse-linked-list-ii/description/

Solution

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverse(ListNode* head) {
        if (!head || !head->next) return head;
        ListNode* ret = reverse(head->next);
        head->next->next = head;
        head->next = NULL;
        return ret;
    }
    
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        if (head == NULL || head->next == NULL || m == n) return head;
        ListNode* fake = new ListNode(-1);
        fake->next = head;
        ListNode* start = fake;
        while (m > 1) {
            start = start->next;
            --m; --n;
        }
        ListNode* end = start;
        while (n > 0) {
            --n;
            end = end->next;
        }
        ListNode* temp = end;
        end = end->next;
        temp->next = NULL;
        ListNode* h = reverse(start->next);
        start->next->next = end;
        start->next = h;
        
        return fake->next;
    }
};