[LeetCode]93. Linked List Cycle II查找链表中环的起始节点

/**
 * 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 == NULL || head->next == NULL) return NULL;
        ListNode *slow = head, *fast = head;
        while (fast->next != NULL && fast->next->next != NULL) { // 判断是否有环
            slow = slow->next;
            fast = fast->next->next;
            if (slow == fast) break;
        }
        if (fast->next == NULL || fast->next->next == NULL) return NULL;
        slow = head;
        while (slow != fast) {
            slow = slow->next;
            fast = fast->next;
        }
        return slow;
    }
};