[LeetCode] Linked List Cycle, Solution

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

 

[Thoughts]

设定两个指针，一个每次走一步，一个每次走两步，如果链表上有环的话，两个指针必定能相遇。否则，则无环

[Code]

   
   

    
    
    
     1 
    
        
    
    bool
    
     hasCycle(ListNode 
    
    *
    
    head) {

    
     2 
    
            
    
    if
    
    (head 
    
    ==
    
     NULL) 
    
    return
    
     
    
    false
    
    ;

    
     3 
    
            ListNode
    
    *
    
     first 
    
    =
    
     head;

    
     4 
    
            ListNode
    
    *
    
     second 
    
    =
    
     head
    
    ->
    
    next;

    
     5 
    
            

    
     6 
    
            
    
    while
    
    (first 
    
    !=
    
     NULL 
    
    &&
    
     second 
    
    !=
    
     NULL)

    
     7 
    
            {

    
     8 
    
                
    
    if
    
    (first 
    
    ==
    
     second) 
    
    return
    
     
    
    true
    
    ;

    
     9 
    
                first 
    
    =
    
     first
    
    ->
    
    next;

    
    10 
    
                second 
    
    =
    
     second
    
    ->
    
    next;

    
    11 
    
                
    
    if
    
    (second 
    
    ==
    
     NULL)

    
    12 
    
                    
    
    return
    
     
    
    false
    
    ;

    
    13 
    
                second 
    
    =
    
     second
    
    ->
    
    next;

    
    14 
    
            }

    
    15 
    
            
    
    return
    
     
    
    false
    
    ;

    
    16 
    
        }