在python中重新排列链表

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

def build_linked_list(nums):
    dummy = ListNode(0)
    curr = dummy
    for num in nums:
        curr.next = ListNode(num)
        curr = curr.next
    return dummy.next

def reorder_list(head):
    if not head or not head.next:
        return head
    
    # 找到链表的中间节点
    slow = head
    fast = head
    while fast.next and fast.next.next:
        slow = slow.next
        fast = fast.next.next
    
    # 反转后半部分链表
    prev = None
    curr = slow.next
    slow.next = None
    while curr:
        next_node = curr.next
        curr.next = prev
        prev = curr
        curr = next_node
    
    # 合并两个链表
    p1 = head
    p2 = prev
    while p2:
        next_p1 = p1.next
        next_p2 = p2.next
        p1.next = p2
        p2.next = next_p1
        p1 = next_p1
        p2 = next_p2
    
    return head