15_reverse_linked_list.cpp

#include <bits/stdc++.h>

struct ListNode {
    int val;
    ListNode *next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
   ListNode(int x, ListNode *next) : val(x), next(next) {}
};

class Solution {
    public:
        ListNode* reverseBetween(ListNode* head, int left, int right) {
            if(!head || left==right) return head; 
            ListNode dummy(0, head);

            ListNode* prev = &dummy; 
            for(int i = 1; i < left; i++)
                prev=prev->next; 

            ListNode* curr = prev->next; 
            ListNode* next; 

            for(int i = 0; i < right - left; i++){
                if (!curr || !curr->next) break;  // Safe guard
                next = curr->next; 

                curr->next = next->next; 
                next->next = prev->next; 
                prev->next = next; 

            }
            return dummy.next;            

        }

    };