单链表中查找环的开始节点

已知壹个链表，尾节点错误的指向了本身，形成了带一段单链表的环链表；头结点已知，链表长度未知，求节点环的开始节点

/* (Step 1) Find the meeting point. This algorithm moves two pointers at 
* different speeds: one moves forward by 1 node, the other by 2. They 
* will always pass the slower one!). If the loop starts k nodes after the 
* start of the list, they will meet at k nodes from the start of the 
* loop. */ 
n1 = n2 = head;  
while (TRUE) {  
    n1 = n1->next;  
    n2 = n2->next->next;  
    if (n1 == n2) {  
        break;   
   }  
}  
/* Find the start of the loop.*/  
n1 = head;  
while (n1->next != n2->next) {  
    n1 = n1->next;  
    n2 = n2->next;  
}  
/*Now n2 points to the start of the loop.*/ 

 

 

 

关于查找环链表的开始节点：
假设单链表长度为L，环形链表长度为K，由于设置了快慢指针，快慢指针一定会在环形链表的某一点相遇，假设该节点距离环形链表开始距离为a.那么可以得到这样的一个结论：慢指针走的距离=L+a，快指针的走的距离=L+a+k*n,此时，让慢指针从头再走，走L的时间正好是快指针走完剩下的路程k-a+(n-1)*k。