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 ;
    // write code here
    struct ListNode *fast = pHead;
    struct ListNode *slow = pHead;
    while(fast && fast -> next)
    {
        fast = fast -> next -> next;
        slow = slow -> next;
        if(fast == slow)
            break;
    } //判断链表是否有环
    //如果链表循环到链表尾部而结束，则无环
    if(fast == NULL || fast -> next == NULL)
        return NULL;
    //找入环节点
    slow = pHead;
    while (slow != fast) 
    {  
        slow = slow->next;  
        fast = fast->next;  
    }
    return  slow;
}