Coding bat practice

• Summer 2020 (COVID-19 pandemic)
• URL: https://codingbat.com/java

Reflections

General points

• I used String.indexOf() == 0 method throughout to check for a matching character or substring at the beginning of the string at hand. In retrospect, however, I should have used String.startsWith() method instead because it would have taken O(1) as opposed to O(n) for indexOf().

parenBit

• My solution to parenBit was based on propagating a state variable hot across recursive calls to indicate if we were inside parenthesis.
• So I had to make an aux function that accepted the state variable as an additional argument and wrapped it inside the main function while initializing it with false.
• I thought , while solving the problem, that it could not be done without maintaining state.
• My solution works, though it has 4 if-else-if branches in addition to the base condition, so can certainly be refactored.
• These habits came from my background in modeling hardware state machines in FPGA, a framework which requires the programmer to be more explicit in specifying their intent.
• Then I looked at the official solution and it blew my mind that they solved the problem in a completely stateless manner.
• There I learned the technique that after reaching the opening parentheses via head pointer, tail pointer can also be advanced to reach for the closing parentheses.
• This looked to me a very elegant solution as it was implemented in only 2 if branches and a base case.
• Interestingly, in such situations, the base case was towards the end after the recursive if conditions fall through and returned the remaining string after the head and tail pointers trimmed it to just what was required.

strCopies

• Similar was the case for strCopies in which I had to count the number of copies, so I thought that it could not be done without maintaining a counter (state).
• It was illuminating to see that the official solution again managed to solve it without maintaining any state.
• They elegantly decremented argument n in recursive calls when a substring match was detected and terminated recursion as soon as n reached zero. Mind = blown!