字符序列

/*    Name:字符排列     Copyright:     Author: Yjy      Date: 08/12/17 17:31    Description: */#include<bits/stdc++.h>using namespace std;int n;long long k=0;int a[20]={0};void f(int);int main(){    cin>>n;    f(1);    printf("%lld",k);    return 0;}void f(int x){      if(x>n){        k++;        return;    }    if(x>=4&&a[x-1]==a[x-3])    {        for(int i=1;i<=3;i++)            if(i!=a[x-2]){                  a[x]=i;                f(x+1);                a[x]=0;            }        return;    }    for(int i=1;i<=3;i++){          a[x]=i;        f(x+1);        a[x]=0;    }}