【Codeforces Round #442 (Div. 2) C】Slava and tanks

【链接】 我是链接,点我呀:)
【题意】

【题解】

【代码】

#include <bits/stdc++.h>
using namespace std;

int n,flag;
vector <int> ans;

int main(){
//	freopen("rush.txt","r",stdin);
	
	scanf("%d",&n);
	flag = n&1;
	if (flag==0){
		for (int i = 1;i <= n;i+=2)
			ans.push_back(i);
		for (int i = 2;i <= n;i+=2)
			ans.push_back(i);
		for (int i = 1;i <= n;i+=2)
			ans.push_back(i);
	}else{
		for (int i = 2;i <= n;i+=2)
			ans.push_back(i);
		for (int i = 1;i <= n;i+=2)
			ans.push_back(i);
		for (int i = 2;i <= n;i+=2)
			ans.push_back(i);
	}
	printf("%d\n",(int) ans.size());
	for (int i = 0;i <(int) ans.size();i++)
		printf("%d ",ans[i]);
	puts("");
	return 0;
}