problem.swift

class Stack<T> {
    private(set) var elements: [T] = []
    
    func push(_ e: T) {
        elements.append(e)
    }
    
    func pop() -> T? {
        defer {
            elements.removeLast()
        }
        
        return elements.last
    }
    
    func peek() -> T? {
        return elements.last
    }
}

func removeAllPairs(_ input: String) -> String {
    var stack = Stack<Character>()
    for c in input {
        if stack.peek() == c {
            _ = stack.pop()
        } else {
            stack.push(c)
        }
    }
    
    return stack.elements.map { String($0) }.joined()
}