【算法合集】双指针
快慢指针
链表判环
slow = head
fast = head
while fast and fast.next:
slow = slow.next # 慢走1步
fast = fast.next.next # 快走2步
if slow == fast:
return True
# 快指针走到末尾，无环
return False
找中点
slow, fast = head, head
while fast and fast.next:
slow = slow.next
fast = fast.next.next
迭代是循环一个个接节点，用 dummy 统一处理头节点，避免单独判断表头。
递归依靠节点之间 next 指针拼接，从头到尾不会新建哑节点。