题解 | #链表中环的入口结点#

 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 * };
 *
 * C语言声明定义全局变量请加上static，防止重复定义
 */
/**
 * 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 ) {
    // write code here
    if(pHead == NULL || pHead->next == NULL)
    {
        return NULL;
    }
    int flag =0;
    struct ListNode* p0;
    struct ListNode* p1;
    struct ListNode* p2;
    struct ListNode* p3;
    p0 = pHead;
    p1 = pHead;
    p2 = pHead;
    p3 = pHead;
    
    while(p0->next->next !=NULL)
    {
        p0 = p0->next;
        p1 = p1->next->next;
        if (p0 == p1)
        {
            flag = 1;
            p3 = p0;
            break;
        }
    }
    if(flag == 1)
    {
        while(p3->next != NULL)
        {
            if(p2 == p3)
            {
                return p3;
            }
            p3 = p3->next;
            p2 = p2->next;

        }
    }
    
    else
    {
        return NULL;
    }
    
    return NULL;
    
}