力扣 141. 环形链表

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 hasCycle(head):
    # 异步
    if not head or not head.next: return False
    s2, s1 = head.next, head
    while s1 != s2:
        if not s2 or not s2.next: return False
        s1 = s1.next
        s2 = s2.next.next
    return True