题解 | 链表中环的入口结点

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