Given a circular linked list, implement an algorithm which returns node at the beginning of the loop.
DEFINITION
Circular linked list: A (corrupt) linked list in which a node’s next pointer points to an earlier node, so as to make a loop in the linked list.
EXAMPLE
DEFINITION
Circular linked list: A (corrupt) linked list in which a node’s next pointer points to an earlier node, so as to make a loop in the linked list.
EXAMPLE
input: A -> B -> C -> D -> E -> C [the same C as earlier]
output: C
分析:這也是一道鏈表的經常討論的題目。有些題目是判斷鏈表中是否有環?方法是使用一個快指針,一個慢指針,兩者必定相遇;有些題目是判斷鏈表是否相交,這個的解法就比較多。
我開始確實也沒有想明白,研究了一下答案。如果A表示慢指針、B表示快指針,當A到達環入口點的時候,假設B領先A k個位置,則他們相遇的地方一定距離入口點k個位置。因爲如果A、B同在起始點,則相遇點也在起始點。B領先k個位置,則相遇點距離起點點反方向k個位置。
代碼:
Node* find_loop_start(Node* head){
if(head==NULL) return;
Node* fast=head,*slow=head;
while(1){
slow=slow->next;
fast=fast->next;
if(fast!=NULL){
fast=fast->next;
if(fast==slow)
break;
}
else
break;
}
if(fast==NULL)
return NULL;
slow=head;
while(1){
if(slow==fast)
return fast;
slow=slow->next;
fast=fast->next;
}
}