[leetcode] 141. Linked List Cycle-优快云博客

本文介绍了两种方法来判断一个链表中是否包含循环：一种是使用哈希表记录节点，另一种是使用双指针技巧。双指针方法通过让快慢指针以不同速度遍历链表，当快指针追上慢指针时，说明链表存在环。

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

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

Solution 1:

Idea: construct a hash table using unordered_map<ListNode*,int>

<span style="font-size:14px;">/**
 * 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) {
        if (!head) return 0;
        ListNode *cur = head;
        unordered_map<ListNode*,int> hs;
        
        while(cur){
            if(++hs[cur]>1){
                return 1;
                break;
            }
            cur = cur->next;
        }
        return 0;
        
    }
};</span>

Solution 2

Idea: use two pointers, one is slow, and the other is fast. 'slow' pointer moves one step each iteration, while 'fast' moves two steps. If the linked list has a cycle, the two pointers will meet each other.

/**
 * 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) {
        if (!head) return false;
        ListNode * slow= head, *fast = head;
        while(fast && fast->next){
            slow = slow->next;
            fast = fast->next->next;
            if (slow==fast)
                return true;
        }
        return false;
    }
};