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?

/**
 * 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) {
        if(!head) return false;
        ListNode * p = head;
        ListNode * q = head;
        
        while(q->next != NULL && q->next->next != NULL){
            p = p->next;
            q = q->next->next;
            if(q == p)
            return true;
      }
        return false;
        
    }
};

1. Use two pointers, walker and runner.
2. walker moves step by step. runner moves two steps at time.
3. if the Linked List has a cycle walker and runner will meet at some
   point.