leetcode : Reverse Linked List II [two pointers]

 

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.

 

Note:
Given m, n satisfy the following condition:
1 ≤ m ≤ n ≤ length of list.

tag: two pointer

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        if(head == null || head.next == null|| m >= n || m <= 0 || n <= 0 ) {
            return head;
        }
        
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        head = dummy;
        
        for(int i = 1; i < m; i++) {
            if(head == null) {
                return null;
            }
            head = head.next;
        }
        
        ListNode preM = head;
        ListNode mNode = head.next;
        ListNode nNode = mNode;
        ListNode nextN = mNode.next;
        
        for(int j = m; j < n; j++) {
            if(nextN == null) {
                return null;
            }
            ListNode tmp = nextN.next;
            nextN.next = nNode;
            nNode = nextN;
            nextN = tmp;
        }
        preM.next = nNode;
        mNode.next = nextN;
       
       return dummy.next;
    }
}