剑指55-链表中环的入口节点

struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(nullptr) {
    }
};

class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead)
    {
        ListNode* fast = pHead;
        ListNode* slow = pHead;
        while (fast && fast->next)
        {
            fast = fast->next->next;
            slow = slow->next;
            if (fast == slow) break;
        }
        if (!fast || !fast->next) return nullptr; //fast到末尾说明不存在
        fast = pHead;
        while (fast != slow)
        {
            fast = fast->next;
            slow = slow->next;
        }
        return fast;
    }
};