回文链表

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def solve():
    def isPalindrome(head):
        if not head or not head.next:
            return True
        slow = fast = head
        while fast.next and fast.next.next:
            slow = slow.next
            fast = fast.next.next
        prev = None
        curr = slow.next
        while curr:
            next_temp = curr.next
            curr.next = prev
            prev = curr
            curr = next_temp
        p1, p2 = head, prev
        while p2:
            if p1.val != p2.val:
                return False
            p1 = p1.next
            p2 = p2.next
        return True
    head = ListNode(1, ListNode(2, ListNode(2, ListNode(1))))
    print(isPalindrome(head))