Odd Even Linked List

Medium Linked List

Bloomberg

Capital One

Facebook

Google

Microsoft

2020-05-16

328. Odd Even Linked List

Question:

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

Note:

The relative order inside both the even and odd groups should remain as it was in the input.
The first node is considered odd, the second node even and so on …

Solution:

Seperate the list into two. One is starting with odd node, another is starting with even. Then combine them together.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode oddEvenList(ListNode head) {
        
        // Curr Node
        ListNode curr = head;
        
        // Dummy Node
        ListNode oddhead = new ListNode(-1);
        ListNode odd = oddhead;
        
        // Dummy Node
        ListNode evenhead = new ListNode(-1);
        ListNode even = evenhead;
        
        // Seprate the Node based on the counter
        int counter = 1;
        while(curr != null) {
            if (counter % 2 == 1) {
                odd.next = curr;
                odd = curr;
            } else {
                even.next = curr;
                even = curr;
            }
            curr = curr.next;
            counter++;
        }
        
        // Avoid Cycle
        even.next = null;
        
        // Link Together
        odd.next = evenhead.next;
        
        return head;
    }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode oddEvenList(ListNode head) {
      
      if(head == null || head.next == null) return head;
      ListNode odd = head, even = head.next;
      ListNode evenDummy = even;
      while(even != null && even.next != null){
          odd.next = even.next;
          odd = odd.next;
          even.next = even.next.next;
          even = even.next;
      }
      odd.next = evenDummy;
      return head;
    
    }
}