https://leetcode.com/problems/valid-parentheses/

20. Valid Parentheses.swift

func isValid(s: String) -> Bool {
    struct Stack<Element> {
        var items = [Element]()
        mutating func push(item: Element) {
            items.append(item)
        }
        mutating func pop() -> Element {
            return items.removeLast()
        }
    }
    
    var stack = Stack<Character>()
    
    for (_, v) in s.characters.enumerate() {
        switch v {
        case "{", "[", "(":
            stack.push(v)
            break
        case "}":
            if stack.items.count == 0 || stack.pop() != "{" {
                return false
            }
            break
        case "]":
            if stack.items.count == 0 || stack.pop() != "[" {
                return false
            }
            break
        case ")":
            if stack.items.count == 0 || stack.pop() != "(" {
                return false
            }
            break
        default:
            continue
        }
    }
    
    if stack.items.count != 0 {
        return false
    }
    
    return true
}