328. Odd Even Linked List

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */


class Solution {
    public ListNode oddEvenList(ListNode head) {
      ListNode odd = new ListNode(0);
      ListNode odd_cur = odd;
      ListNode even = new ListNode(0);
      ListNode even_cur = even;
      
      ListNode cur = head;
      
      int count = 1;
      while( cur != null){
        if(count % 2 == 1){
          odd_cur.next = cur;
          odd_cur = odd_cur.next;
        }else{
          even_cur.next = cur;
          even_cur = even_cur.next;
        }
        cur = cur.next;
        count++;
      }
      
      odd_cur.next = even.next;
      even_cur.next = null;
      return odd.next;
        
    }
}

 

Given a singly linked list, group all odd nodes together followed by the even nodes. Please note here we are talking about the node number and not the value in the nodes.

You should try to do it in place. The program should run in O(1) space complexity and O(nodes) time complexity.

Example 1:

Input: 1->2->3->4->5->NULL
Output: 1->3->5->2->4->NULL

Example 2:

Input: 2->1->3->5->6->4->7->NULL
Output: 2->3->6->7->1->5->4->NULL