- 判断链表是否有环.
用快慢指针，注意点在于考虑清楚链表长度为0，1，2，3的情况。while循环判断条件建议用fast != slow
def hasCycle(self, head):
"""
:type head: ListNode
:rtype: bool
"""
if not head or not head.next:
return False
slow = head.next
fast = head.next.next
while fast != slow:
if not fast or not fast.next:
return False
slow = slow.next
fast = fast.next.next
return True
- 找到入环点。注意点在于初始化的时候让slow先走一步，fast先走两步，始终保持1:2的关系。
def detectCycle(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
if not head or not head.next:
return None
slow = head.next
fast = head.next.next
while fast != slow:
if not fast or not fast.next:
return None
fast = fast.next.next
slow = slow.next
cur = head
while cur != slow:
cur = cur.next
slow = slow.next
return cur
