链表环判断

bool IsLoop(Node* node) {
    node = node->next;
    Node* fast = node;
    Node* slow = node;
    while(fast) {
        fast = fast->next;
        if (!fast) return false;
        fast = fast->next;
        if (!fast) return false;

        // fast 成功走了两步
        if (fast == slow) return true;

        slow = slow->next;
    }
}

快慢指针 fast走两步 slow走一步

由于 slow 走完一圈的时候 fast可以走两圈 所以会收敛