P7593 - Resistance

内存限制：128 MB 时间限制：2 S 标准输入输出

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You are given a cactus with

n vertices and

m edges. There is a 1 Ohm resistor on every edge.

Let

f(s,t) be the equivalent resistance between the vertex

s and the vertex

t .

Print

∑1≤s<t≤nf(s,t) .

Note: a cactus (sometimes called a cactus tree) is a connected graph in which any two simple cycles have at most one vertex in common. Equivalently, it is a connected graph in which every edge belongs to at most one simple cycle.

The first line contains one integer

T(1≤T≤7) - the number of the test cases.

For each test case, the first line contains two integers

n,m(1≤n≤2×105,1≤m≤4×105) .

Each of the following

m lines contains three integers

u,v(1≤u≠v≤n) .

For each test case, print the answer modulo

109+7 . That is,the answer can be represented as a fraction

a/b where

(a,b)=1 , output

c(0≤c<109+7) such that

bc≡a(mod109+7) .

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7593: Resistance

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