2021-01-21

136.只出现一次的数字

class Solution:
    def singleNumber(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        a = 0
        for num in nums:
            a = a ^ num
        return a

141.环形链表I

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        # 快慢指针法
        if head:
            slow, fast = head, head.next
            while fast:
                slow, fast = slow.next, fast.next
                if fast:
                    fast = fast.next
                if fast is slow:
                    return True
        return False
        
        """哈希法
        hash_set = set()
        while head:
            if head in hash_set:
                return True
            hash_set.add(head)
            head = head.next
        return False
        """

142.环形链表II

def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        # 首先初始化快指针和慢指针，确保快指针走的路的长度是慢指针长度的2倍
        if head and head.next:
            fast = head.next.next
            slow = head.next
        else:
            return None  # 说明无环

        # 进行循环，首先让快指针和慢指针第一次相遇
        while fast:
            if fast != slow:

                # 快指针走两步
                if fast.next:
                    fast = fast.next.next
                else:
                    return None  # 说明无环

                # 慢指针走一步
                slow = slow.next
            else:
                detection = head
                while detection != slow:  # 此时由于slow和fast是一样的，用哪个都行
                    slow = slow.next
                    detection = detection.next

                return detection