链表中环的入口节点

题目描述

给一个链表，若其中包含环，请找出该链表的环的入口结点，否则，输出null。

第一想法

• 通过指针地址是否出现重复来判断.
• key value对.

AC代码

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
 
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead)
    {
        ListNode* p = pHead;
        map<ListNode* , int> m;
        while(p){
            if(++m[p] == 2)
                return p;
            p = p -> next;
        }
        return p;
    }
};

看了讨论区

通过快慢指针的形式

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

class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead)
    {
        ListNode *fast = pHead, *slow = pHead;
        while (fast && fast->next != NULL) {
            fast = fast->next->next;
            slow = slow->next;
            if (fast == slow) {
                break;
            }
        }
        
        if (fast == NULL || fast->next == NULL) {
            return NULL;
        }
        
        fast = pHead;
        while (fast != slow) {
            fast = fast->next;
            slow = slow->next;
        }
        
        return fast;
    }
};