When is a decision tree most commonly used?

An organisation that owns dozens of shopping malls wants to create a machine learning product that will use facial recognition to identify customers. What is the main challenge of developing such a model?

What is the time complexity for breadth-f‌irst search algorithm? Given, d = depth.

XGBoos it provides interfaces for which of the following options? i) C++ ii) R iii) Python iv) Julia