G
题目描述
Takahashi wants to gain muscle, and decides to work out at AtCoder Gym.
The exercise machine at the gym has N buttons, and exactly one of the buttons is lighten up. These buttons are numbered 1 through N. When Button i is lighten up and you press it, the light is turned off, and then Button ai will be lighten up. It is possible that i=ai. When Button i is not lighten up, nothing will happen by pressing it.
Initially, Button 1 is lighten up. Takahashi wants to quit pressing buttons when Button 2 is lighten up.
Determine whether this is possible. If the answer is positive, find the minimum number of times he needs to press buttons.
Constraints
2≤N≤105
1≤ai≤N
输入
Input is given from Standard Input in the following format:
N
a1
a2
:
aN
输出
Print −1 if it is impossible to lighten up Button 2. Otherwise, print the minimum number of times we need to press buttons in order to lighten up Button 2.
样例输入
3
3
1
2
样例输出
2
提示
Press Button 1, then Button 3.
漏点：只考虑到可能出现n一直点an的情况，没有考虑到可能有循环环
J
题目描述
Snuke has N dogs and M monkeys. He wants them to line up in a row.
As a Japanese saying goes, these dogs and monkeys are on bad terms. (“ken’en no naka”, literally “the relationship of dogs and monkeys”, means a relationship of mutual hatred.) Snuke is trying to reconsile them, by arranging the animals so that there are neither two adjacent dogs nor two adjacent monkeys.
How many such arrangements there are? Find the count modulo 109+7 (since animals cannot understand numbers larger than that). Here, dogs and monkeys are both distinguishable. Also, two arrangements that result from reversing each other are distinguished.
Constraints
1≤N,M≤105
输入
Input is given from Standard Input in the following format:
N M
输出
Print the number of possible arrangements, modulo 109+7.
样例输入
2 2
样例输出
8
提示
We will denote the dogs by A and B, and the monkeys by C and D. There are eight possible arrangements: ACBD, ADBC, BCAD, BDAC, CADB, CBDA, DACB and DBCA.
漏点：res1*=i;不对
。。。res1=res1*i;就对了