代码不是这么写吗？

#include <bits/stdc++.h>
#define int long long
using namespace std;
const int mod = 1000000007ll, N = 1000010;
int n, m, u[N], v[N], f[N], ans;
signed main()
{
	cin >> n >> m;
	for (int i = 1; i <= m; i++)
		scanf("%lld%lld", u + i, v + i);
	if (n > 100)
	{
		f[0] = 1;
		for (int i = 1; i <= n; i++)
			f[i] = (f[i - 1] << 1ll) % mod;
		for (int i = 1; i <= n; i++)
			ans += (f[n - i] * f[max(0ll, i - 2)] % mod) * (n - i + 1) % mod;
		printf("%lld", ans);
		return 0;
	}
	printf("%lld", dfs(1));
}