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) {
        //fast low
        //if have a cycle,fast==slow
        ListNode* slow=hasCycle(pHead);
        if(slow==nullptr)
            return nullptr;//no cycle
        //be sure there is cycle
        ListNode* fast=pHead;
        while(fast!=slow){
            fast=fast->next;
            slow=slow->next;
        }//why
        return slow;
    }
    ListNode* hasCycle(ListNode* head){
        if(head==nullptr) return nullptr;
        ListNode* fast=head;
        ListNode* slow=head;
        while(fast!=nullptr&&fast->next!=nullptr){
            fast=fast->next->next;
            slow=slow->next;
            if(fast==slow)
                return slow;
        }
        return nullptr;
    }
};

why:

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