class Solution {
    func isValid(_ s: String) -> Bool {
        var stack: [Character] = []

        for c in s {
            switch c {
                case "(":
                    stack.append(")")
                case "{":
                    stack.append("}")
                case "[":
                    stack.append("]")
                default:
                    if (stack.popLast() != c) {
                        return false
                    }
            }
        }

        return stack.isEmpty
    }
}