20. Valid Parentheses

class Solution {
    func isValid(_ s: String) -> Bool {
        if [")", "]", "}"].contains(s.first) { return false }
        if ["(", "[", "{"].contains(s.last) { return false }
        var str: String = ""
        for c in s {
            if str.last == "(" && c == ")" || str.last == "[" && c == "]" || str.last == "{" && c == "}" {
                str.removeLast()
            } else {
                str.append(c)
            }
        }
        return str.count == 0
    }
}