Useless use of Stack

The title is modelled, obviously, after the notorious useless use of cat.

Consider the following FizzBuzz challenge:

Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

Open brackets must be closed by the same type of brackets. Open brackets must be closed in the correct order. Example 1:

Input: s = "()" Output: true Example 2:

Input: s = "()[]{}" Output: true Example 3:

Input: s = "(]" Output: false

My first instinct is to use a counter. A commonly contrived solution is to use a stack.

A Stack solution would look something like this:

type Stack struct {
    store []interface{}

func (s *Stack) push(obj interface{}) {
    s.store = append(s.store, obj)

func (s *Stack) pop() (interface{}, bool) {
    if len(s.store) == 0 {
        return nil, false
    last := len(s.store) - 1
    ret := s.store[last]
    s.store = s.store[:last]
    return ret, true

func isBalancedAll(inp string) bool {
    parensStack := Stack{}
    angleStack := Stack{}
    curlyStack := Stack{}

    for _, char := range inp {
        switch char {
        // Angle
        case '[':
        case ']':
            _, found := angleStack.pop()
            if !found {
                return false
        // Parens
        case '(':
        case ')':
            _, found := parensStack.pop()
            if !found {
                return false
        // Curly
        case '{':
        case '}':
            _, found := curlyStack.pop()
            if !found {
                return false

    for _, stack := range []Stack{angleStack, parensStack, curlyStack} {
        _, found := stack.pop()
        if found {
            return false
    return true

The stack solution is a solution in search of a problem. This could very easily be a counter. We're pushing to the stack(++), popping(–), and checking for emptiness(==0). We're not even interested in ordering - the stack can only ever contain one type of thing at any given moment, all we care for is the count. In fact, we could use a queue - FIFO access. And all we'd care about is whether or not the queue is empty.

In my opinion, the stack's raison d'etre is LIFO access. A consequence of LIFO access is that one essentially trasverses items in reverse while popping the stack. Hence the use of the stack to evaluate arithmetic expressions in order to observe precedence rules. Or while converting decimal to binary and so forth. These use-cases specifically need LIFO access.

Some other uses, such as the one above, are painfully contrived. And a horrible choice of example when teaching the stack.

Date: 2023-02-11

Author: Brian Kamotho