Shares of Nu Holdings (NYSE: NU) stock gained 10% in October, according to data from S&P Global Market Intelligence. The Brazil-based bank is benefiting from increased investor enthusiasm in the banking sector as interest rates go down.

The better bank stock

Nu is not your typical bank, and some investor optimism is also likely to be in anticipation of an excellent third-quarter earnings report. However, Nu stock has started to fall in November, which isn't unusual before earnings. Nu stock had hit an all-time high, and that something leads to a light correction. Third-quarter earnings will be released Nov. 13.

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Nu is an all-digital bank serving Brazil, Mexico, and Colombia. Although it's relatively young, it has already become one of the most popular banks in Brazil, with 56% of the adult population as members as of the end of the second quarter. That percentage has climbed every quarter and should be higher in the third quarter.

The percentage increase in members in Brazil is slowing down, but it's still impressive; customer count increased 20% year over year in the second quarter. Its other markets are growing faster, but they're still small. In Mexico it added more than 1 million customers in the second quarter for a total of 7.8 million, and it topped 1 million customers in Colombia, reaching 1.3 million.

Aside from new members, it's also raising more revenue through increased engagement among current members. Nu offers a large selection of financial services, and new members continue to upgrade and add services, leading to increased revenue per active user.

All this growth is spiraling upward into higher sales and profit. Nu is still in high growth mode, with a 65% revenue increase year over year in the second quarter, and net income rose from $225 million last year to $487 million this year. But it still has a massive growth runway.

Follow Buffett into this stock

Warren Buffett's Berkshire Hathaway has a small position in Nu stock, at least relative to some of its other stocks. In some ways, it's the classic Buffett stock, because it's a bank stock with lots of cash, growing profits, and expanding revenue streams. At the same time, it's a rare tech-focused stock in the Berkshire Hathaway portfolio.

If the third-quarter report is anything like its past few reports, expect the stock to jump on the release. If it doesn't, don't worry; buy it for the long-term opportunity.