LeetCode 141 Linked List Cycle (set, map, a pointer speed)

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set heavy sentence:

/**
 * 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) {
        unordered_set<ListNode*> st;
        while(head!=NULL)
        {
            if(st.count(head))  return true;
            st.insert(head);
            head = head->next;
        }
        return false;
    }
};

map：

/**
 * 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) {
        unordered_map<ListNode*, int> mp;
        while(head!=NULL)
        { 
            if(mp[head]==2) return true;
            mp[head]++;
            head = head->next;
        }
        return false;
    }
};

Pointer speed:

Characterized by a single list of each node knows only the next node, so that only determines whether or not a pointer list containing ring.

• If the list does not contain the ring, then the pointer will eventually encounter a null pointer or null list coming to an end, this is better said, can determine this list does not contain ring.

• But if the list contains a loop, then the pointer will be caught in an infinite loop, because the annular array is not null pointer as the tail node.

Classic solution is to use two hands, a run fast, a slow runner.

• If the ring is not contained, the pointer to run faster eventually encounter null, list description no ring;
• If the ring containing, ultra-fast pointer will eventually slow the pointer around, and slow to meet this pointer indicates the list containing ring.

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