题解 | 小红的双生排列

package main

import (
    "fmt"
)

func main() {
    var n int64
    fmt.Scan(&n)
     mod := 1e9 + 7
     var l int64
     v1  := factorial(n/2, mod)
    if n%2 == 0 {
        l = v1 * v1%int64(mod)
        l = 2 * l%int64(mod)
    } else {
        v2 := factorial((n+1)/2, mod)
        l = v2 * v1%int64(mod)
    }
    fmt.Println(l)  
}

func factorial(n int64, mod float64) int64 {
    var v int64 = 1
    for i:=int64(2); i <= n; i++ {
        v *= i
        v %= int64(mod)
    }
    return v
}