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) {
    }
};
*/
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        //快慢指针
    
        //判断有环无环
        ListNode* slow = pHead;
        for(int i = 0;i < 10000;i++)
        {
            if(slow == nullptr) return nullptr;slow = slow->next;
        }
            
            
        
        slow = pHead->next;
        ListNode* quick = pHead->next->next;
       
        while(slow != quick)  //有环就会相遇
        {
            slow = slow->next;
            quick = quick->next->next;
        }
        
        quick = pHead;
        while(quick != slow)
        {
            quick = quick->next;
            slow = slow->next;
        }

        return slow;
    }
};