力扣 142. 环形链表II

前置

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

解法

def detectCycle(head):
    # 异步
    if not head or not head.next:
        return None
    s1, s2 = head, head
    while s2 and s2.next:
        s1 = s1.next
        s2 = s2.next.next
        if s1 == s2: break
    if not s2 or not s2.next: return
    s2 = head
    while s1 != s2:
        s1 = s1.next
        s2 = s2.next

    return s2