Checking Balanced Parentheses (an Application of Stacks)

In this lesson you will learn how to check that brackets are balanced using a stack in Kotlin, a practical application of the stack data structure. This is for anyone searching for "Kotlin balanced parentheses stack".

With a stack (last in, first out) you can push opening brackets such as ( and [, then check each closing bracket against whatever is on top. That is how bracket matching works.

The sample code keeps the opening bracket for each closing bracket in a Map called pairs, and uses an ArrayDeque to push with addLast() on an opening bracket and check the top of the stack on a closing one. If the stack is empty once everything has been processed, the brackets are balanced.

A common stumbling block for beginners is handling several kinds of bracket at once (round, square, and curly mixed together). Counting openings and closings is not enough - you also have to confirm they nest in the right order.

The same idea is used behind editor auto-completion and code syntax checking. Plenty of similar tools check that HTML opening and closing tags line up correctly.

📖 Reference code

fun isBalanced(str: String): Boolean {
    val stack = ArrayDeque<Char>()
    val pairs = mapOf(')' to '(', ']' to '[', '}' to '{')
    for (ch in str) {
        if (ch in "([{") {
            stack.addLast(ch)
        } else if (ch in ")]}") {
            if (stack.isEmpty() || stack.removeLast() != pairs[ch]) return false
        }
    }
    return stack.isEmpty()
}

fun main() {
    println(isBalanced("(a[b]{c})"))
    println(isBalanced("(a[b)]"))
}

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