暴力写的，不知道是不是TLE了：

#include<bits/stdc++.h>
using namespace std;
const int MAXN = 2e5 + 5;
int a[MAXN];
int main()
{
	//freopen(".in", "r", stdin);
	//freopen(".out", "w", stdout);
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	int n, cnt;
	cin >> n;
	cnt = 2 * n - 1;
	for(int i = 1; i <= cnt; i ++)
		cin >> a[i];
	while(n != 1)
	{
		n --;
		cnt = 2 * n - 1;
		int aa = a[1], b = a[2], c = a[3];
		for(int i = 1; i <= cnt; i ++)
		{
			int k = aa + b + c;
			int maxx = max(aa, max(b, c));
			int minn = min(aa, min(b, c));
			a[i] = k - maxx - minn;
			aa = b, b = c, c = a[i + 3];
		}
	}
	cout << a[1];
	return 0;
}