142. Linked List Cycle II

O(n) time O(1) space

/**
 * 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) {
        auto slow(head), fast(head);
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
            if (slow == fast)
                break;
        }
        if (!fast || !fast->next) return nullptr; // 有可能没有cycle
        while (head != slow) {
            head = head->next;
            slow = slow->next;
        }
        return head;
    }
};