019-141-Linked List Cycle 判断链表是否有环

Problem

Given a linked list, determine if it has a cycle in it.

Follow up:

Can you solve it without using extra space?

Solution

解决方法：龟兔赛跑。快慢指针。

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
bool hasCycle(ListNode *head) {
bool is_circle = false;
ListNode* slow = head;
ListNode* fast = head;
while(slow != NULL && fast !=NULL){
slow = slow->next;
if(fast->next != NULL){
fast = fast->next->next;
}else{
break;
}
if(slow == fast){
is_circle = true;
break;
}
}
return is_circle;
}
};

解法2：使用哈希表。建立一个哈希表，从链表开始逐个访问节点，当发现有重复的节点的时候，可以判断链表为有环。

More

可以找到链表环开始的节点并返回该节点吗？参考我的博文020-142-Linked List Cycle II