142. 环形链表 II

Posted 2022-11-28 爱吃榴莲的喵星人

文章目录

• 一、题目描述
• 二、做题思路
• 三、题目代码

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一、题目描述

题目链接

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二、做题思路

思路一：公式证明

思路二：当不知道推导的公式时
相遇时一定在圈里，它是尾节点
以相交链表的思想去做

思路三：

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三、题目代码

思路一：公式证明

/**
 * Definition for singly-linked list.
 * struct ListNode 
 *     int val;
 *     struct ListNode *next;
 * ;
 */
struct ListNode *detectCycle(struct ListNode *head) 
    struct ListNode * fast = head;
    struct ListNode * slow = head;
    struct ListNode * meet = NULL;
    while(fast && fast->next)
    
        fast=fast->next->next;
        slow=slow->next;
        if(fast == slow)
        
            meet = fast;
           while(head != meet)
           
             head=head->next;
             meet=meet->next;
           
           return meet;
        
    
    return NULL;

思路二：

/**
 * Definition for singly-linked list.
 * struct ListNode 
 *     int val;
 *     struct ListNode *next;
 * ;
 */
 struct ListNode *getIntersectionNode(struct ListNode *headA, struct ListNode *headB,struct ListNode *tail) 
    struct ListNode * listA = headA;
    struct ListNode * listB = headB;
    int lenA=1;int lenB=1;
    while(listA!=tail)
    
        listA=listA->next;
        lenA++;
    
    while(listB!=tail)
    
        listB=listB->next;
        lenB++;
    
    if(listA != listB)
    
        return NULL;
    
    int gap=abs(lenA-lenB);
    struct ListNode * longlist = headA;
    struct ListNode * shortlist = headB;
    if(lenA < lenB)
    
        longlist = headB;
        shortlist = headA;
    
    while(gap--)
    
        longlist=longlist->next;
    
    while(longlist != shortlist)
    
        longlist=longlist->next;
        shortlist=shortlist->next;
    
    return shortlist;

struct ListNode *detectCycle(struct ListNode *head) 
    struct ListNode * fast = head;
    struct ListNode * slow = head;
    struct ListNode * meet = NULL;
    while(fast && fast->next)
    
        slow=slow->next;
        fast=fast->next->next;
        if(fast==slow)
        
            meet=slow;
            struct ListNode * newhead = meet->next;
            struct ListNode * p=getIntersectionNode(newhead,head,meet);
            return p;
        
    
   return NULL;

思路三：

/**
 * Definition for singly-linked list.
 * struct ListNode 
 *     int val;
 *     struct ListNode *next;
 * ;
 */
 struct ListNode *getIntersectionNode(struct ListNode *headA, struct ListNode *headB) 
    struct ListNode * listA = headA;
    struct ListNode * listB = headB;
    int lenA=1;int lenB=1;
    while(listA->next)
    
        listA=listA->next;
        lenA++;
    
    while(listB->next)
    
        listB=listB->next;
        lenB++;
    
    if(listA != listB)
    
        return NULL;
    
    int gap=abs(lenA-lenB);
    struct ListNode * longlist = headA;
    struct ListNode * shortlist = headB;
    if(lenA < lenB)
    
        longlist = headB;
        shortlist = headA;
    
    while(gap--)
    
        longlist=longlist->next;
    
    while(longlist != shortlist)
    
        longlist=longlist->next;
        shortlist=shortlist->next;
    
    return shortlist;

struct ListNode *detectCycle(struct ListNode *head) 
    struct ListNode * fast = head;
    struct ListNode * slow = head;
    struct ListNode * meet = NULL;
    while(fast && fast->next)
    
        slow=slow->next;
        fast=fast->next->next;
        if(fast==slow)
        
            meet=slow;
            struct ListNode * newhead = meet->next;
            meet->next=NULL;
            struct ListNode * p=getIntersectionNode(newhead,head);
            return p;
        
    
   return NULL;

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