链表5

忘是亡心i 2022-05-25 00:21 49阅读 0赞

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