题解 | #串#

package main

import (
	"fmt"
)

func main() {
   var n int
   fmt.Scan(&n)
   var noSet int64= 1
   var setUS int64= 0
   var setS int64= 0
   var mod int64= 1000000007
   var ans int64= 0
   for i := 0; i < n; i++ {
        setUS = (setS + setUS*26%mod)%mod
        setS = (noSet + setS*25%mod)%mod
        noSet = noSet*25%mod
        ans = (ans + setUS)%mod
   }
   fmt.Println(ans)
}