Sword To Offer 055 - 链表中环的入口结点

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
private:
    unordered_set<ListNode*> book;
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        while(pHead != nullptr) {
            if(book.find(pHead) == book.end()) {
                book.insert(pHead);
                pHead = pHead->next;
            } else {
                return pHead;
            }
        }
        return pHead;
    }
};