142. Linked List Cycle II

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* FloydDetectCycle(ListNode* head){
        if(head == NULL)
            return NULL;
        ListNode* slow = head;
        ListNode* fast = head;

        while(slow != NULL && fast != NULL)
        {
            fast = fast -> next;
            if(fast == NULL)
                return NULL;
            fast = fast -> next;
            slow = slow -> next;

            if(slow == fast)
                return slow;
        }

        return NULL;
    }

    ListNode *detectCycle(ListNode *head) {
        if(head == NULL)
            return NULL;

        ListNode* intersection = FloydDetectCycle(head);
        if(intersection == NULL)
            return NULL;

        ListNode* slow = head;

        while(slow != intersection)
        {
            slow = slow -> next;
            intersection = intersection -> next;
        }

        return slow;    
    }
};