【LeetCode】 142. Linked List Cycle II C语言

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode *detectCycle(struct ListNode *head) {
    if(head==NULL || head->next==NULL) return NULL;
    struct ListNode *fast=head;
    struct ListNode *slow=head;
    
    while(1)
    {
        if(fast==NULL || fast->next==NULL) return NULL; //遇到null了，说明不存在环
        slow=slow->next;
        fast=fast->next->next;
        
        if(fast == slow) break; //第一次相遇在Z点
    }
    slow=head; //slow从头开始走
    while(slow!=fast)
    {
        slow=slow->next;
        fast=fast->next;
    }
    return slow;
}