[leetcode] 141. Linked List Cycle

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;
}
};