String extension for balancing by "open" and "close" substrings. String balanced algorithm using Swift

BalancedStrings.swift

protocol StackProtocol {
    associatedtype ElementType
    
    var isEmpty: Bool { get }
    
    mutating func pop() -> ElementType?
    mutating func push(_ value: ElementType)
}


struct Stack<T>: StackProtocol {
    fileprivate var array: [T] = []
    
    //MARK: StackProtocol
    typealias ElementType = T
    
    var isEmpty: Bool {
        return array.isEmpty
    }
    
    mutating func pop() -> T? {
        return array.popLast()
    }
    
    mutating func push(_ value: T) {
        array.append(value)
    }
}


extension String {
    func isBalanced(open:Set<Character>,close:Set<Character>) -> Bool {
        var stack = Stack<Character>()
        let xs = self.filter{open.contains($0) || close.contains($0)}
        guard false == xs.isEmpty else {return false}
        for char in xs {
            if open.contains(char) {
                stack.push(char)
            }
            else {
                if stack.isEmpty {return false}
                if let topChar = stack.pop(),
                    open.contains(topChar) && !close.contains(char) {
                    return false
                }
            }
        }
        return stack.isEmpty
    }
}

//true
"((()))()".isBalanced(open: Set("("), close: Set(")"))
"[][[]][]".isBalanced(open: Set("["), close: Set("]"))
"([])[()]()[]".isBalanced(open: Set("(["), close: Set(")]"))
"()[]([])".isBalanced(open: Set("[("), close: Set("])"))
"()[]([])".isBalanced(open: Set("["), close: Set("]"))
"()[]([])".isBalanced(open: Set("("), close: Set(")"))
"⎨⎦⎣⎝⎛⎠«⎞<⟩»>⟨⎬".isBalanced(open: Set("⎨⎣⎝⎛«<⟨"), close: Set("⎬⎦⎠⎞»>⟩"))

//false
")(".isBalanced(open: Set("("), close: Set(")"))
"][".isBalanced(open: Set("["), close: Set("]"))
"()".isBalanced(open: Set("["), close: Set("]"))
"[]".isBalanced(open: Set("("), close: Set(")"))

//reverted
")(][".isBalanced(open: Set(")]"), close: Set("[("))

improved with Set for O(n) complexity