Linked List Cycle

llc.cpp

/**
 * 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) {
        auto p1 = head;
        auto p2 = head;
        
        // Loop Invariant: 
        while (p1 != NULL && p2 != NULL && p2->next != NULL) {
            p1 = p1 -> next;
            p2 = p2 -> next -> next;
            if (p1 == p2)
                return true;
        }
        
        return false;
    }
};

// Note:
// 1. Don't miss the 'p2 != NULL' in the while condition