c++

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead)
    {
        if (pHead == NULL || pHead->next == NULL) return NULL;
        ListNode* slower = pHead, *faster = pHead, *entry = pHead;
        while(faster->next && faster->next->next) {
            faster = faster->next->next;
            slower = slower->next;
            if (faster == slower) {
                while(slower != entry) {
                    entry = entry->next;
                    slower = slower->next;
                }
                return entry;
            }
        }
        return NULL;
    }
};