Reverse a portion of a linked list (from position m to n)?

ListNode ReverseBetween(ListNode head, int m, int n) {

if (head == null || m == n) return head;

ListNode dummy = new ListNode(0);

dummy.next = head;

ListNode prev = dummy;

// Move prev to one before m-th node

for (int i = 1; i < m; i++) prev = prev.next;

ListNode start = prev.next;

ListNode then = start.next;

// Reverse the sublist

for (int i = 0; i < n - m; i++) {

Follow on:

start.next = then.next;

then.next = prev.next;

prev.next = then;

then = start.next;

return dummy.next;

Explanation:

Use a dummy node to simplify edge cases. Reverse nodes between m and n by changing

pointers.