2454: Permutation
内存限制:128 MB
时间限制:3.000 S
评测方式:文本比较
命题人:
提交:0
解决:0
题目描述
bobo has a permutation p1,p2,…,pn of 1,2,…,n.
Knowing m extra constraints of form pai<pbi, bobo wanna count the number of different permutations modulo (109+7).
It is guaranteed that there is at least one such permutation.
输入
The input consists of several tests. For each tests:
The first line contains n,m (1≤n≤40,0≤m≤20). Each of the following m lines contain 2 integers ai,bi(1≤ai,bi≤n).
输出
For each tests:
A single number denotes the number of permutations.
样例输入 复制
3 1
1 2
3 2
1 2
2 3
样例输出 复制
3
1