Leetcode 92. Reverse Linked List II

/**
 * 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(m == n) {
            return head;
        }
        ListNode* start = nullptr;
        ListNode* end = nullptr;
        ListNode* left = nullptr;
        ListNode* right = nullptr;

        ListNode* pre = nullptr;
        ListNode* current = head;
        ListNode* next = nullptr;
        int diff = n - m + 1;
        m--;
        while(m) {
            pre = current;
            current = current->next;
            m--;
        }
        start = pre;
        right = current;
        while(diff) {
            next = current->next;
            current->next = pre;
            pre = current;
            current = next;
            diff--;
        }
        left = pre;
        end = current;
        right->next = end;
        if(start) {
            start->next = left;
            return head;
        }
        else {
            return left;
        } 
    }
};