Reverse Linked List

lc 206

public class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode prev = null;

        while(head != null){
            ListNode temp = head.next;
            head.next = prev;
            prev = head;
            head = temp;
        }
        return prev;
    }
}

Node 在那个position， i就从什么开始，i < n 的话， node就停在 n上

lc92

Reverse Linked List II

Reverse a linked list from position m to n. Do it in-place and in one-pass.

For example:
Given 1->2->3->4->5->NULL, m = 2 and n = 4,

return 1->4->3->2->5->NULL.

public class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        if (m >= n || head == null) {
            return head;
        }

        ListNode dummy = new ListNode(0);
        dummy.next = head;
        head = dummy;

        for (int i = 0; i < m - 1; i++) {
            if (head == null) {
                return null;
            }
            head = head.next;
        }

        ListNode premNode = head;
        ListNode mNode = head.next;
        ListNode nNode = mNode, postnNode = mNode.next;
        for (int i = m; i < n; i++) {
            if (postnNode == null) {
                return null;
            }
            ListNode temp = postnNode.next;
            postnNode.next = nNode;
            nNode = postnNode;
            postnNode = temp;
        }
        mNode.next = postnNode;
        premNode.next = nNode;

        return dummy.next;
    }
}