链表5

1、题目:给定一个链表，判断它是否是循环链表

        Given a linked list, determine if it has a cycle in it.

        Follow up:
                Can you solve it without using extra space?

2、解答：该题目是链表是单链表，只有一个指针，所以循环一定在表尾，不存在循环后面还有结点。因此解法是 让一个结点每次走2步，另外一个结点走一步，若是循环链表，必定有相遇的时候。

3、C++代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode *curr = head;
        ListNode *currTwoStep = head;
        
        if(!curr|| !curr->next)
            return false;
        
        currTwoStep = curr->next;
        
        while(curr && currTwoStep){
            if(curr == currTwoStep)     //快的结点，遇到了走的慢的结点
                return true;
            
            if(!currTwoStep->next)
                return false;
            
            currTwoStep = currTwoStep->next->next;    //快的结点走2步，慢的结点走1步
            curr = curr->next;
        }
        return false;
    }
};

python 代码

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        
        slow,fast = head,head
        while fast and fast.next:
            slow,fast = slow.next,fast.next.next
            if slow is fast:
                return True
        return False