Problem

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

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

Code

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
static int var = [](){
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if(head==NULL)
            return false;
        ListNode* fast = head;
        ListNode* slow = head;
        while(fast->next!=NULL && fast->next->next!=NULL){
            fast=fast->next->next;
            slow=slow->next;
            if(slow==fast)
                return true;
        }
        return false;
    }
};

Result

141. Linked List Cycle.png