[leetcode] #141 Linked List Cycle

[leetcode] #141 Linked List Cycle

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

Follow up:

Can you solve it without using extra space?

用O(1)空间，判断单链表是否存在环

解题思路：

1、fast指针走两步， low指针走一步

2、循环后，若有环则存在fast指针追上low指针那一刻。

至于为何走两步，而不走3步，4步

fast指针每次都比low指针多走一步，若有环，假设一开始差距为n。则每次都缩减一步，当差距为0时，自然两者重合了，走3步，4步， 若n不能整除（3-1）或（4-1）需更多圈才能重合。

[code]/**
 * 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) {
        ListNode *low = head;
        ListNode *fast = head;
        while (low && fast && fast->next) {
            low = low->next;
            fast = fast->next->next;
            if (low == fast) {
                return true;
            }
        }
        return false;
    }
};