已知壹個連結串列，尾節點錯誤的指向了本身，形成了帶一段單鏈表的環連結串列；頭結點已知，連結串列長度未知，求節點環的開始節點

/* (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.*/ 

關於查詢環連結串列的開始節點：
假設單鏈表長度為L，環形連結串列長度為K，由於設定了快慢指標，快慢指標一定會在環形連結串列的某一點相遇，假設該節點距離環形連結串列開始距離為a.那麼可以得到這樣的一個結論：慢指標走的距離=L+a，快指標的走的距離=L+a+k*n,此時，讓慢指標從頭再走，走L的時間正好是快指標走完剩下的路程k-a+(n-1)*k。