面试题 02.06. 回文链表

这里不需要判断奇偶性，1->2->1会==> 1->2<-1
1->3->3->1==>1->3 3<-1
public class Num0206回文链表 {
    public boolean isPalindrome(ListNode head) {
        if (head == null || head.next == null) return true;
        ListNode fast = head, slow = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        ListNode pre = null;
        while (slow != null) {
            ListNode next = slow.next;
            slow.next = pre;
            pre = slow;
            slow = next;
        }
        ListNode cur = head;
        while (pre != null) {
            if (pre.val != cur.val) return false;
            pre = pre.next;
            cur = cur.next;
        }
        return true;
    }
}