LeetCode101: 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 *detectCycle(ListNode *head) {
		if (head == nullptr || head->next == nullptr) {
			return nullptr;
		}

		// tortoise move 1 step
		auto slow = head->next;

		// hare move 2 steps
		auto fast = head->next->next;

		while (slow != fast) {
			if (slow == nullptr || fast == nullptr || fast->next == nullptr) {
				return nullptr;
			}
			slow = slow->next;
			fast = fast->next->next;
		}
		slow = head;
		while (slow != fast) {
			slow = slow->next;
			fast = fast->next;
		}
		return slow;
	}
};