LintCode 102. Linked List Cycle

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

O(1)空间要求，用快慢指针，若有环，快慢指针必回相交，这是一个追击问题。

若无空间要求，可以用Hashmap

/**
 * Definition of singly-linked-list:
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *        this->val = val;
 *        this->next = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param head: The first node of linked list.
     * @return: True if it has a cycle, or false
     */
    bool hasCycle(ListNode * head) {
        if (head == NULL) {
            return false;
        }
        
        ListNode * slow = head, * fast = head->next;
        
        while (fast != NULL && fast->next != NULL) {
            if (slow == fast) {
                return true;
            }
            slow = slow->next;
            fast = fast->next->next;
        }
        
        return false;
    }
};