题解 | 牛牛学数列6

package main

import (
	"fmt"
)

func main() {
	var n int
	fmt.Scan(&n)

	if n == 1 {
		fmt.Print(0)
	}
	if n == 2 || n == 3 {
		fmt.Print(1)
	}
	a := make([]int, 3)
	a[0], a[1], a[2] = 0, 1, 1
	for i := 4; i <= n; i++ {
		a[0], a[1], a[2] = a[1], a[2], a[2]+2*a[1]+a[0]
	}
	fmt.Print(a[2])
}