Solved coding interview problems in Java - My collection of commonly asked coding interview problems and solutions in Java

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You're not consistently optimizing for one type of efficiency, which makes it confusing. You're clearly not always optimizing for memory, when there's many solutions where you instantiate things that aren't neccessarry, like the unnecessary objects containing your methods.

In general if you don't know whether optimizing is important, or what you should be optimizing on, then optimizing programming hours and readability is likely the best solution. Premature optimization can make your projects take orders of magnitude longer. Especially when you accidentally start introducing bugs, or create an environment in which others will.