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 ) {
    // write code here
    if(pHead == NULL || pHead->next == NULL){
        return NULL;
    }
    struct ListNode *fast = pHead;
    struct ListNode *slow = pHead;
    while(fast != NULL && fast->next !=NULL){
        slow = slow ->next;
        fast = fast->next->next;
        if(slow == fast){
            struct ListNode *ptr1 = pHead;
            struct ListNode *ptr2 = slow;
            while(ptr1 != ptr2){
                ptr1 = ptr1->next;
                ptr2 = ptr2->next;
            }
            return ptr1;
        }
    }
    return NULL;
}