一、双指针（Two Pointers）

 二、力扣141题

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        if head is None:
            return False
        slow = head
        fast = head
        while(fast is not None and fast.next is not None):
            slow = slow.next
            fast = fast.next.next
            if slow == fast:
                return True
        return False

三、力扣881题

class Solution(object):
    def numRescueBoats(self, people, limit):
        """
        :type people: List[int]
        :type limit: int
        :rtype: int
        """
        if len(people) == 0:
            return 0
        people.sort()
        i = 0
        j = len(people) - 1
        res = 0
        while(i <= j):
            if people[i] + people[j] <= limit:
                i += 1
            j -= 1
            res += 1
        return res