Find the starting point of the Loop of LinkedList | Linked List Cycle II

 https://leetcode.com/problems/linked-list-cycle-ii/

 

soln 1:

use hash map hash nodes

if node appears which already is in hash table that is the first element with cycle

 if all node are distinct then return null.

soln 2:

class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if(head==NULL || head->next==NULL) return NULL;
        ListNode *slow=head;
        ListNode *fast=head;
        ListNode *entry=head;
        while(fast->next  && fast->next->next){
            slow=slow->next;
            fast=fast->next->next;
            if(slow==fast){
                while(slow !=entry ){
                    slow=slow->next;
                    entry=entry->next;
                }
                return entry;
            }
        }
        return NULL;
    }
};