/
1540B.go
77 lines (72 loc) · 1.29 KB
/
1540B.go
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package main
import (
"bufio"
. "fmt"
"io"
)
// github.com/EndlessCheng/codeforces-go
func CF1540B(_r io.Reader, out io.Writer) {
in := bufio.NewReader(_r)
const mod int64 = 1e9 + 7
const mod2 = (mod + 1) / 2
pow := func(x, n int64) (res int64) {
res = 1
for ; n > 0; n >>= 1 {
if n&1 > 0 {
res = res * x % mod
}
x = x * x % mod
}
return
}
min := func(a, b int) int {
if a < b {
return a
}
return b
}
var n, v, w int
Fscan(in, &n)
dis := make([][]int, n)
for i := range dis {
dis[i] = make([]int, n)
for j := range dis[i] {
dis[i][j] = 1e9
}
}
for i := 1; i < n; i++ {
Fscan(in, &v, &w)
v--
w--
dis[v][w] = 1
dis[w][v] = 1
}
for k := range dis {
for i := range dis {
for j := range dis {
dis[i][j] = min(dis[i][j], dis[i][k]+dis[k][j])
}
}
}
f := make([][]int64, n+1)
for i := range f {
f[i] = make([]int64, n+1)
f[0][i] = 1
}
for i := 1; i <= n; i++ {
for j := 1; j <= n; j++ {
f[i][j] = (f[i-1][j] + f[i][j-1]) * mod2 % mod
}
}
ans := int64(0)
for _, d := range dis {
for v, r := range dis {
for w, dvw := range r[:v] {
commonDis := (d[v] + d[w] - dvw) / 2
ans += f[d[v]-commonDis][d[w]-commonDis]
}
}
}
Fprint(out, ans%mod*pow(int64(n), mod-2)%mod)
}
//func main() { CF1540B(os.Stdin, os.Stdout) }