题解 | 链表中环的入口结点

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 * };
 */
/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 *
 * 
 * @param pHead ListNode类 
 * @return ListNode类
 */
struct ListNode* EntryNodeOfLoop(struct ListNode* pHead ) 
{
    if(pHead==NULL)return NULL;
    if(pHead->next==NULL)return NULL;
    struct ListNode* slow=pHead;
    struct ListNode* fast=pHead;
    while(fast!=NULL&&fast->next!=NULL&&fast->next->next!=NULL)
    {
        fast=fast->next->next;
        slow=slow->next;
        if(fast->next==slow->next)
        {
            fast=pHead;
            while(fast!=slow)
            {
                fast=fast->next;
                slow=slow->next;
            }
            return fast;
        } 

    }
    return NULL;
}