@jianchao-li

/**
* 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) {
       ListNode *dummy = new ListNode(0), *pre = dummy;
       dummy -> next = head;
       for (int i = 1; i < m; i++) {
           pre = pre -> next;
       }
       ListNode* cur = pre -> next;
       for (int i = 0; i < n - m; i++) {
           ListNode* move = cur -> next;
           cur -> next = move -> next;
           move -> next = pre -> next;
           pre -> next = move;
       }
       return dummy -> next;
   }
};

@user9187Q
Can someone explain to me why we need the dummy pointer or local object? Why can’t we just have ListNode* pre = head ? Does that because of the pointer shadow copying?
@pow25
It simply easy the case that we need to reverse the list from beginning, if just use pre = head, then we need to do extra work for the new head