标签：style blog http color 使用 sp strong on div
Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
使用双指针fast和slow，fast每次走两步，slow每次走一步，如果fast追上slow则存在环，否则不存在。
1 bool hasCycle(ListNode *head) 2 { 3 ListNode *slow = head, *fast = head; 4 while (fast != NULL) 5 { 6 slow = slow->next; 7 fast = fast->next; 8 if (fast != NULL) 9 fast = fast->next; 10 if (fast != NULL && fast == slow) 11 return true; 12 } 13 14 return false; 15 }
Linked List Cycle ii则是要求出环出现的那个节点。
在fast追上slow时，fast比slow多走了nC，n>=1, C为圆的周长。如下图：
slow走的路程：lenA + x，fast走的路程：LenA + nC + x
因为fast走的路程是slow的两倍：2(lenA + x) = lenA + nC + x
于是, nC = lenA + x, 即lenA = nC - x。
那么相遇时，让slow指向head，fast和slow走一样的步长，最终相遇在Join处。
1 ListNode *detectCycle(ListNode *head) 2 { 3 ListNode *fast = head, *slow = head; 4 5 while (fast != NULL) 6 { 7 slow = slow->next; 8 fast = fast->next; 9 if (fast != NULL) 10 fast = fast->next; 11 if (fast != NULL && fast == slow) 12 break; 13 } 14 15 if (fast == NULL) 16 return NULL; 17 18 slow = head; 19 while (slow != fast) 20 { 21 slow = slow->next; 22 fast = fast->next; 23 } 24 25 return slow; 26 }
leetcode. Linked List Cycle && Linked List Cycle ii
标签：style blog http color 使用 sp strong on div
原文地址：http://www.cnblogs.com/ym65536/p/4141424.html
