leetcode 142

判断出来有环之后，根据等量关系可以找到入环点。

class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if ( !head || !head->next ) return nullptr;
        ListNode *slow = head, *fast = head, *ptr = head;
        while ( fast && fast->next ) {
            slow = slow->next, fast = fast->next->next;
            if ( slow == fast ) {
                for ( ; ptr != slow; ptr = ptr->next, slow = slow->next );
                return ptr;
            }
        }
        return nullptr;
    }
};