LeetCode Reverse Linked List II

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.

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

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
            if (head==null) {
                return null;
            }
            if(n==m){
                return head;
            }
            ListNode temp=head,doRe = null,second=null,first=null,last=null;
            int i=1;
            
            while (temp!=null) {
                if (i==m-1) {
                    first=temp;
                }
                if (i==m) {
                    doRe=temp;
                }
                if (i==n) {
                    last=temp;
                    second=temp.next;
                    if(first!=null) first.next=last;
                    last.next=null;
                    
                    break;
                }
                ++i;
                temp=temp.next;
            }
            reverse(doRe);
            doRe.next=second;
            if (m>1) {
                return head;
            }else {
                return last;
            }
            
    }
    private void reverse(ListNode head) {
        ListNode pre = null,curr,next;

        curr=head;
        if (head==null) {
            return;
        }
        while (curr!=null) {
            next=curr.next;
            curr.next=pre;
            pre=curr;
            curr=next;
        }
    }
}