leetcode: Linked List Cycle

http://oj.leetcode.com/problems/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?

思路：

这个问题是面试时很常见的问题，但是要求写代码的比较少。最简单的方法就是两个指针，一个每次走一步，一个每次走两步，等赶上了就说明有环。

class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (NULL == head) {
            return false;
        }
        
        ListNode *curr = head, *next = head->next;
        
        while (true) {
            curr = curr->next;
            
            int steps = 0;
            
            while ((next != NULL) && (steps < 2)) {
                next = next->next;
                ++steps;
            }
            
            if (NULL == next) {
                return false;
            }
            
            if (curr == next) {
                return true;
            }
        }
    }
};