Another task

Solution.swift

import Foundation

/*
 (())()
 (()
 )((())
 */

func isValid(_ str: String) -> Bool {
    var count = 0
    for c in str {
        switch c {
        case "(": count += 1
        case ")": count -= 1
        default: count = -1; break
        }
        guard count >= 0 else { break }
    }
    return count == 0
}

print(isValid("(())()"))
print(isValid("(()"))
print(isValid(")((())"))