day7--the entry node of the ring in the linked list

 

1. Use the "fast and slow pointer" to find the first meeting point;

2. One starts from the head node, and the other starts from the first meeting point;

3. When you meet again is the entry point.

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
#include <cstddef>
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        if(pHead==NULL || pHead->next==NULL) return NULL;
        ListNode *slow=pHead, *fast=pHead;
        while(fast!=NULL && fast->next!=NULL){
            slow=slow->next;
            fast=fast->next->next;
            if(fast==slow) break;
        }
        if(fast==NULL || fast->next==NULL) return NULL;
        ListNode *src=pHead;//指向头节点
        ListNode *tmp=slow;//指向第一次相遇的节点
        while(src!=tmp){
            src=src->next;
            tmp=tmp->next;
        }
        return src;
    }
};

reference: 

(285 messages) The entry node of the ring in the linked list---C++ implementation-CSDN Blog