Nullpointerexception when I perform depth first search
What is a NullPointerException, and how do I fix it?
I added a comment to  but something strange is happening. The end code reads "); but SO decided to change that to ";);.    I did not add a ; in front of the ).  To further explain what's happening here, the following:  "http://example.com/." http://example.com/. "http://example.com." http://example.com. "http://example.com" http://example.com   renders as: $\sum_{k=1}^nH_k = (n+1)H_n-n$. Why? What is the formula for Real vs Apparent Depth, when not looking perpendicular to the interface?   I know the formula $$\frac{\text{real depth}}{\text{apparent depth}}=n$$   but I'm not looking for this. Beautiful table samples I'm interested in learning about some nice-looking table samples. Colors can make a huge difference, and just providing some color to headers, background, or borders can dramatically change presentation. Any samples you're proud to share?  (BTW here's a sample I like, but unfortunately, not TeX...) LED as power generator Matlab arrays operation I have built a forecasting model for a company. Since it is dedicated to practical usage, I prefer to use the relative error parameter (like MAPE, SMAPE, &amp; MASE) as a measurement for my model performance and display it on the dashboard (the percentage error is easier to understand by user). I found an interesting reference about the criteria of a good forecast result based on MAPE from Lewis (1982):  ...but in my case, I can't use MAPE for some dataset because there is some zero actual demand so it can't be used as denominator in MAPE. So, I use SMAPE &amp; MASE instead, but I can't find any good references about good forecast result criteria based on SMAPE &amp; MASE, nothing like above. So, I'd really appreciate any help/suggestion about it :), Thank You! Reference: Lewis, C. D., 1982. Industrial and Business Forecasting Methods. London: Butterworths. Since drush 9 (drupal 8.4 requirement), alias @sites does not seem to be implemented anymore (or yet ?).  $ drush @sites cr The alias @sites could not be found.   Is this a way to get the same behavior that does not consist of typing my 300 alias in a config file ? Computing second partial derivative with polar coordinates Consider the polar coordinates with $x=r\cos\theta$ and $y=r\sin\theta$.  I can show using the chain rule that $$\frac{\partial}{\partial x} = \frac{x}{r} \frac{\partial}{\partial r} -\frac{y}{r^2} \frac{\partial}{\partial \theta}$$   What is the method to compute $\dfrac{\partial^2}{\partial x^2}$? I don't know how to do it. Can a PC be a lich? The heading says it all, can a PC be a lich? If so, how? The DMG hints that the Book of Vile Darkness contains the secrets of how; there is nothing official otherwise. What can I do when getting “This question body does not meet our quality standards”? Why was M87 targeted for the Event Horizon Telescope instead of Sagittarius A*? The first image of a black hole has been released today, April 10th, 2019. The team targeted .  Why didn't the team target  at the center of our own galaxy? Intuitively, it would seem to be a better target as it is closer to us. How to convert image file data in a byte array to a Bitmap? Performance impact of updating target columns with same values ON CONFLICT This is a question about the inner-workings of Postgres (v10) and performance.  Given a table, github_repos, with a multi-column unique index on org_id and github_id columns, is there any performance difference (or other issues to be aware of) between the two bulk upsert operations below? The difference is that in the first query, the org_id and github_id columns are included in the UPDATE, whereas in the second query they are not. Since the UPDATE runs ON CONFLICT, the updated values of org_id and github_id will be the same as the old values, but those columns are included in the UPDATE because the underlying library I am using is designed that way. I'm wondering if its safe to use as-is or whether I should be explicitly excluding those columns in the UPDATE.  Query #1:  INSERT INTO "github_repos" ("org_id","github_id","name") VALUES (1,1,'foo') ON CONFLICT (org_id, github_id) DO UPDATE SET "org_id"=EXCLUDED."org_id","github_id"=EXCLUDED."github_id","name"=EXCLUDED."name" RETURNING "id"   Query #2:  INSERT INTO "github_repos" ("org_id","github_id","name") VALUES (1,1,'foo') ON CONFLICT (org_id, github_id) DO UPDATE SET "name"=EXCLUDED."name" RETURNING "id"   github_repos table:        Column       |       Type        | Collation | Nullable  -------------------+-------------------+-----------+----------+  id                | bigint            |           | not null |  org_id            | bigint            |           | not null |  github_id         | bigint            |           | not null |  name              | character varying |           | not null |  Indexes:     "github_repos_pkey" PRIMARY KEY, btree (id)     "unique_repos" UNIQUE, btree (org_id, github_id) Easy way to model spiral engraving (knurling) that is usually used on metal for better grip? What is the easiest way to model this diamond grip pattern into a cylinder? All Stack Exchange data dumps The original ClearBits link to the September 2011 data dump has broken. How can I access it and other previous data dumps? The Internet Archive only has . Are drops in Google ranking common after switching to https? About a week ago I made the switch to https on one of my sites. I'm positive I did everything right. Properly setup 301 redirects from http to https versions. Added and verified https version in Webmaster Tools. Updated all internal links. Submitted new sitemap.  Despite that, many of my pages (about 40%) lost their rankings almost immediately. Those that dropped went from page 1 to nowhere. I can still see them indexed if I search for the URL, but otherwise it's like Google doesn't know they exist.  SSL Labs shows an "A" when I run their test and I have no manual actions showing in Webmaster Tools.  This is really frustrating. I don't actually need SSL, but Google finally scared me into it. Now I'm regretting the switch and contemplating going back to non-secured URLs.  Anything else I could have missed? How do I grep for lines containing either of two words, but not both? I'm trying to use grep to show only lines containing either of the two words, if only one of them appears in the line, but not if they are in the same line.  So far I've tried  grep pattern1 | grep pattern2 | ... but didn't get the result I expected. Ignoring somebody screws up the avatar list When I ignore a user, the avatar list looks weird.    The blank line beneath the row with the avatar of the ignoree is quite annoying.    I use Google Chrome on OS X.  There was a  tagged  but it still looks weird. normal subgroups of Alternating group Klein's $4$ group, $\{1\}$ and $A_4$ are three normal subgroups of $A_4$. I want to know about any other normal subgroups of $A_4$. Do these exist?