Leetcode #20: Takes in a string of braces (any mix of parentheses, square brackets, and curly braces) and returns whether they are validly delimited.

ValidParentheses.swift

import Foundation

enum ParenthesisType: CaseIterable {
    case round
    case square
    case curly
    
    var open: Character {
        switch self {
        case .round:
            return "("
        case .square:
            return "["
        case .curly:
            return "{"
        }
    }
    
    var close: Character {
        switch self {
        case .round:
            return ")"
        case .square:
            return "]"
        case .curly:
            return "}"
        }
    }
    
    static func from(character: Character) -> ParenthesisType? {
        for type in ParenthesisType.allCases {
            if type.open == character || type.close == character {
                return type
            }
        }
        return nil
    }
}

private var stack = [ParenthesisType]()

func isValid(_ s: String) -> Bool {
    for character in s {
        guard let type = ParenthesisType.from(character: character) else {
            return false
        }
        if character == type.open {
            stack.append(type)
        }
        else if stack.last == type {
            stack.removeLast()
        }
        else {
            return false
        }
    }
    return stack.isEmpty
}