20. Valid Parentheses

20. Valid Parentheses.swift

class Solution {
    func isValid(_ s: String) -> Bool {
        var stack = [Character]()
        let closingPairs: [Character:Character] = [ "{":"}", "[":"]", "(":")" ]
        let openBrackets = Set<Character>(closingPairs.keys)
        for char in s {
            if openBrackets.contains(char) {
                stack.append(char)
            } else {
                guard let last = stack.popLast() else {
                    return false
                }
                if closingPairs[last] != char {
                    return false
                }
            }
        }
        return stack.isEmpty
    }

}

let sol = Solution()
let example = ""
print(sol.isValid(example))