Ludwig Wittgenstein, the author of Philosophical Investigations and the Tractatus Logico-Philosophicus, a don at Cambridge, and one of the most important philosophers of the 20th century, was a Catholic Austrian Jew who fought as an officer for the Imperial Austro-Hungarian Army in WWI and was decorated for bravery.
When Germany took Austria in the Anschluss, the wealthy Wittgenstein family became subject to Germany’s Nuremberg Laws. With three Jewish grandparents, Wittgenstein was to lose Austrian citizenship and become a Jewish non-citizen under those discriminatory laws. Wittgenstein’s baptism and Catholic upbringing did not exempt him.
Wittgenstein’s family offered the Nazis their substantial wealth for classification as German citizens. Hitler agreed, though supposedly he only approved such adjustments in twelve cases. The Wittgensteins turned over 1700 kg. of gold to the Nazis. In 1939, Ludwig was naturalized in the UK.
Hitler and Wittgenstein were the same age and attended the same grammar school in Linz at the same time, though they were two classes apart. Hitler was held back one year and Wittgenstein skipped a grade.
Their interactions, if any, are the subject of speculation.