【To Do】LeetCode 142. Linked List Cycle II

LeetCode 142. Linked List Cycle II

Solution1：我的答案
这道题多次遇到，牢记此解法
这道题要深思一下，快指针和慢指针的速度对比不同，会产生什么不同的结果？

/**
 * 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) 
            return NULL;
        struct ListNode *slow = head, *fast = head;
        bool cycle = false;
        while (slow->next && fast->next && fast->next->next) {
            slow = slow->next;
            fast = fast->next->next;
            if (slow == fast) {
                cycle = true;
                break;
            }
        }
        if (!cycle)
            return NULL;
        struct ListNode* cur = head;
        while (cur != slow) {
            cur = cur->next;
            slow = slow->next;
        }
        return cur;
    }
};