List Cycle

• Time : $O(n)$
• Space : $O(1)$

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
ListNode* Solution::detectCycle(ListNode* A) {
    ListNode* slow = A, *fast = A;
    while(fast->next and fast->next->next) {
        slow = slow->next;
        fast = fast->next->next;
        if(slow == fast) break;
    }
    if(fast->next and fast->next->next) {
        ListNode* runner = A;
        while(runner != slow) {
            slow = slow->next;
            runner = runner->next;
        }
        return runner;
    }
    return NULL;
}