Medium/Reorder List.java

/**
 * 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 void reorderList(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode secondHalf = reverse(slow.next);
        slow.next = null;
        ListNode firstHalf = head;
        while (firstHalf != null && secondHalf != null) {
            ListNode firstNext = firstHalf.next;
            ListNode secondNext = secondHalf.next;
            firstHalf.next = secondHalf;
            secondHalf.next = firstNext;
            firstHalf = firstNext;
            secondHalf = secondNext;
        }
    }

    private ListNode reverse(ListNode node) {
        ListNode curr = node;
        ListNode prev = null;
        while (curr != null) {
            ListNode next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }
}