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Exercise 1: Implementing a stack using a list

Consider the following interface for the Stack ADT:

interface Stack {
    fun push(item: Int)
    fun pop(): Int
    fun peek(): Int
    fun isEmpty(): Boolean
}

Write a class ListStack that implements this interface using a MutableList as its underlying data store.

Note that you can also override a function to be able to print your ListStack directly:

    /* Return a string representing this ListStack. */
    override fun toString(): String {
        // TODO
    }

Here is a main function you could use to test your code:

fun main() {
    val s = ListStack()
    s.push(2025)
    s.push(9)
    s.push(26)
    
    println("Elements present in stack: ")
    println(s)
    
    println("" + s.pop() + " popped from stack")
    println("Top element is: " + s.peek())

    println("Elements present in stack: ")
    println(s)
}

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Exercise 2: Validating proper nesting

We can say that a sequence of operations (for example, lock/unlock requests) is “properly nested” we complete requests in the reverse order that we start them. For example, the following are properly nested sequences (assuming the first occurrence of a letter marks the start and the second marks the end for that pair):

• AA
• ABBA
• ABCCBA
• ABBCCA

However, the following are not properly nested sequences:

• ABAB
• ABCACB

How could you use a stack to determine whether a given input string is properly nested? Try to write some code to solve this!

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