for (int i = 20; i >= 0; i--) {
        if (fa[x][i] != fa[y][i]) {
            ans = std::max(ans, std::max(f_max[x][i], f_max[y][i]));
            x = fa[x][i];
            y = fa[x][i]; // 犯这种错误是人啊，倍增求lca都能写错
        }
    }

https://www.luogu.com.cn/record/190978864