Check a loop in a list with line…

Effectively check a loop in a list with linear cost
    
 =============   
 1. Analysis
 =============
 
    typical case 1:
             1->2->3->4->5->6->1   ( loop in header, K = 0, N = 6)
    typical case 2:
             1->2->3->4->5->6->3   ( loop in node 3, K = 2, N = 4)
    
    Two pointers, one with each step 1, the other with each step2
    Suppose step = K + x ( or say cost)
             K + x = 2*(K+x) - N
             ==>  x = N - k
 =============   
 2. Algorithm
 =============
    
    typedef struct Node Node;
    struct Node
    {
        void* data;
        Node* p;
    };
    boolean check_loop(const Node* p)
    {
        
        Node* pTemp = p;
        
        Node* p1 = p?(p->p):NULL;
        Node* p2 = (p1)?(p1->p):NULL;
        
        for(;p2;p1 = p1->p,p2 = (pTemp =(p2->p))?(pTemp->p):NULL )
        {
            if ( p1 == p2)
            {
                return true;
            }
        }
        return false;
    }