LeetCode刷题(5)【链表】【环形链表II】(C语言)

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode *detectCycle(struct ListNode *head) {
    struct ListNode* slow = head;
    struct ListNode* fast = head;
    while(fast && fast->next)
    {
        fast = fast->next->next;
        slow = slow->next;
        //找到相遇点
        if(fast == slow)
        {
            //相等即为相遇点
            struct ListNode* meet =  slow;
            //一个指针从meet走，一个指针从head走，他们会在入口点相遇
            while(head != meet)
            {
                head = head->next;
                meet = meet->next;
            }
            return meet;
        }
    }
    return NULL;
}