LeetCode Reverse Nodes in k-Group

  1 /**
  2  * Definition for singly-linked list.
  3  * public class ListNode {
  4  *     int val;
  5  *     ListNode next;
  6  *     ListNode(int x) {
  7  *         val = x;
  8  *         next = null;
  9  *     }
 10  * }
 11  */
 12 public class Solution {
 13     public ListNode reverseKGroup(ListNode head, int k) {
 14 
 15         if (head==null) {
 16             return null;
 17         }
 18         if (k == 1) {
 19             return head;
 20         }
 21 
 22         ListNode temp=head;
 23         ListNode doRe = head, nextList = null, first = null, last = null;
 24         int i=1;
 25 
 26         while (temp != null) {
 27 
 28             if (i % k == 0) {
 29                 if (i == k) {
 30                     first = reverseBetween(doRe, i / k, i);
 31 
 32                 } else {
 33                     last.next = reverseBetween(doRe, 1, k);
 34                 }
 35                 last = doRe;
 36                 temp = doRe.next;
 37                 doRe = doRe.next;
 38             } else {
 39             //    last = temp;
 40                 temp = temp.next;
 41             }
 42 
 43             ++i;
 44         }
 45         return first==null?doRe:first;
 46 
 47     }
 48     
 49     
 50     public ListNode reverseBetween(ListNode head, int m, int n) {
 51         if (head==null) {
 52             return null;
 53         }
 54         if(n==m){
 55             return head;
 56         }
 57         ListNode temp=head,doRe = null,second=null,first=null,last=null;
 58         int i=1;
 59 
 60         while (temp!=null) {
 61             if (i==m-1) {
 62                 first=temp;
 63             }
 64             if (i==m) {
 65                 doRe=temp;
 66             }
 67             if (i==n) {
 68                 last=temp;
 69                 second=temp.next;
 70                 if(first!=null) first.next=last;
 71                 last.next=null;
 72 
 73                 break;
 74             }
 75             ++i;
 76             temp=temp.next;
 77         }
 78         reverse(doRe);
 79         doRe.next=second;    
 80 
 81         if (m>1) {
 82             return head;
 83         }else {
 84             return last;
 85         }
 86 
 87     }
 88     private void reverse(ListNode head) {
 89         ListNode pre = null,curr,next;
 90 
 91         curr=head;
 92         if (head==null) {
 93             return;
 94         }
 95         while (curr!=null) {
 96             next=curr.next;
 97             curr.next=pre;
 98             pre=curr;
 99             curr=next;
100         }
101     }
102 
103 
104 
105 }