Reorder List.java

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param head: The head of linked list.
     * @return: void
     */
    public void reorderList(ListNode head) {  
        // write your code here
        if (head == null || head.next == null) {
            return;
        }
        
        ListNode midNode = getMid(head);
        ListNode rev = reverse(midNode.next);
        midNode.next = null;
        
        merge(head, rev);
    }
    
    public ListNode getMid(ListNode head) {
        ListNode slow = head, fast = head.next;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        return slow;
    }
    
    public ListNode reverse(ListNode head) {
        ListNode newNode = null;
        while (head != null) {
            ListNode temp = head.next;
            head.next = newNode;
            newNode = head;
            head = temp;
        }
        return newNode;
    }
    
    public void merge(ListNode head, ListNode rev) {
        int index = 0;
        ListNode dummy = new ListNode(0);
        while (head != null && rev != null) {
            if (index % 2 == 0) {
                dummy.next = head;
                head = head.next;
            } else {
                dummy.next = rev;
                rev = rev.next;
            }
            dummy = dummy.next;
            index++;
        }
        if (head != null) {
            dummy.next = head;
        } else {
            dummy.next = rev;
        }
    }
    
}