August 23, 2020

Go Interview Question #3 - Write a function that checks if brackets are balanced

Another very common interview question, is to check if brackets are balanced in a string. If you have never done any stack questions before, this can be tricky. But, once you see how a stack can be used, it becomes very straightforward.

Let’s look at what balanced brackets look like: “(())()”, “((()))”, “()”…

Unbalanced brackets: “(((”, “())”, “(()”…

A stack is like a can of Pringles, the last chip that was put into it, is the first to come out.

Yummy yummy!

Since Golang does not implement stack structure out of the box, we can use a slice to simulate its behavior. To simulate the the push() function, we will simply use built-in append(). For pop() we will have to use s = s[:len(s)-1].

Now, the strategy should be more clear. If we see an opening bracket, we add it to the “stack”. If we see a closing one, we try to pop it off the “stack”. If at any point we cannot pop the chip off the stack, we know the brackets are not balanced. if there are any brackets left over in the stack after the loop is finished, the brackets are also not balanced. The resulting code:

There is a more complicated variation of this problem, where the brackers can be any of the following: {([])} At first, this sounds much more complicated, but it really isn’t. The only additional thing we need to do when we are trying to pop the last value, is check whether the last bracket in our “stack” is a match for our popping bracket. The easiest way is to use a map, resulting in:

© Ilya Nemtsev 2019

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