剑指offer之判断链表是否包含环

1 问题

判断链表是否包含环

2 思路

2个指针，一个指针走一步，一个指针走2步，如果相遇则有，反之无。

3 代码实现

  

   

    

     

    
    

     

      #include <stdio.h>
     
    
   

    

     

    
    

     

      #include <stdlib.h>
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     

      #define true 1
     
    
   

    

     

    
    

     

      #define false 0;
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     

      typedef struct node
     
    
   

    

     

    
    

     

      {
     
    
   

    

     

    
    

     
 int value;
     
    
   

    

     

    
    

     
 struct node *next;
     
    
   

    

     

    
    

     

      }Node;
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     

      /*
     
    
   

    

     

    
    

     

       *判断链表是否有环
     
    
   

    

     

    
    

     

       */
     
    
   

    

     

    
    

     

      int isCircleList(Node *head)
     
    
   

    

     

    
    

     

      {
     
    
   

    

     

    
    

     
 if (head == NULL)
     
    
   

    

     

    
    

     

       {
     
    
   

    

     

    
    

     
 return false;
     
    
   

    

     

    
    

     

       }
     
    
   

    

     

    
    

     

       Node *first = NULL;
     
    
   

    

     

    
    

     

       Node *second = NULL;
     
    
   

    

     

    
    

     

       first = head;
     
    
   

    

     

    
    

     

       second = head;
     
    
   

    

     

    
    

     
 while (second != NULL && (second->next) != NULL && (second->next->next != NULL))
     
    
   

    

     

    
    

     

       {
     
    
   

    

     

    
    

     

       first = first->next;
     
    
   

    

     

    
    

     

       second = second->next->next;
     
    
   

    

     

    
    

     
 if (first == second)
     
    
   

    

     

    
    

     

       {
     
    
   

    

     

    
    

     
 return true;
     
    
   

    

     

    
    

     

       }
     
    
   

    

     

    
    

     

       }
     
    
   

    

     

    
    

     
 return false;
     
    
   

    

     

    
    

     

      }
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     

      int main()
     
    
   

    

     

    
    

     

      {
     
    
   

    

     

    
    

     

       Node *head = NULL;
     
    
   

    

     

    
    

     

       Node *node1 = NULL;
     
    
   

    

     

    
    

     

       Node *node2 = NULL;
     
    
   

    

     

    
    

     

       Node *node3 = NULL;
     
    
   

    

     

    
    

     

       Node *node4 = NULL;
     
    
   

    

     

    
    

     

       Node *node5 = NULL;
     
    
   

    

     

    
    

     

       Node *node6 = NULL;
     
    
   

    

     

    
    

     

       Node *node7 = NULL;
     
    
   

    

     

    
    

     

       head = (Node *)malloc(sizeof(Node));
     
    
   

    

     

    
    

     

       node1 = (Node *)malloc(sizeof(Node));
     
    
   

    

     

    
    

     

       node2 = (Node *)malloc(sizeof(Node));
     
    
   

    

     

    
    

     

       node3 = (Node *)malloc(sizeof(Node));
     
    
   

    

     

    
    

     

       node4 = (Node *)malloc(sizeof(Node));
     
    
   

    

     

    
    

     

       node5 = (Node *)malloc(sizeof(Node));
     
    
   

    

     

    
    

     

       node6 = (Node *)malloc(sizeof(Node));
     
    
   

    

     

    
    

     

       node7 = (Node *)malloc(sizeof(Node));
     
    
   

    

     

    
    

     
 if (head == NULL || node1 == NULL || node2 == NULL || node3 == NULL
     
    
   

    

     

    
    

     

       || node4 == NULL || node5 == NULL || node6 == NULL || node7 == NULL)
     
    
   

    

     

    
    

     

       {
     
    
   

    

     

    
    

     
 printf("malloc fail\n");
     
    
   

    

     

    
    

     
 return false;
     
    
   

    

     

    
    

     

       }
     
    
   

    

     

    
    

     
 // node7<-node6 <-node5
     
    
   

    

     

    
    

     
 // | |
     
    
   

    

     

    
    

     
 //head->node1->node2->node3->node4
     
    
   

    

     

    
    

     

       head->value = 0;
     
    
   

    

     

    
    

     

       head->next = node1;
     
    
   

    

     

    
    

     

       node1->value = 1;
     
    
   

    

     

    
    

     

       node1->next = node2;
     
    
   

    

     

    
    

     

       node2->value = 2;
     
    
   

    

     

    
    

     

       node2->next = node3;
     
    
   

    

     

    
    

     

       node3->value = 3;
     
    
   

    

     

    
    

     

       node3->next = node4;
     
    
   

    

     

    
    

     

       node4->value = 4;
     
    
   

    

     

    
    

     

       node4->next = node5;
     
    
   

    

     

    
    

     

       node5->value = 5;
     
    
   

    

     

    
    

     

       node5->next = node6;
     
    
   

    

     

    
    

     

       node6->value = 6;
     
    
   

    

     

    
    

     

       node6->next = node7;
     
    
   

    

     

    
    

     

       node7->value = 7;
     
    
   

    

     

    
    

     

       node7->next = node2;
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
 int result = isCircleList(head);
     
    
   

    

     

    
    

     
 if (result)
     
    
   

    

     

    
    

     

       {
     
    
   

    

     

    
    

     
 printf("list have circle\n");
     
    
   

    

     

    
    

     

       }
     
    
   

    

     

    
    

     
 else
     
    
   

    

     

    
    

     

       {
     
    
   

    

     

    
    

     
 printf("list do not have circle\n");
     
    
   

    

     

    
    

     

       }
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
 return true;
     
    
   

    

     

    
    

     

      }
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
 
     
    
   

    

     

    
    

     
 
     
    
  
 

4 运行结果

list have circle
 

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原文链接：chenyu.blog.csdn.net/article/details/87748768