BalancedBracketValidator.swift

func validate(input: String) -> Bool {
    let brackets: [Character:Character] = ["[":"]","(":")","{":"}"]
    var stack: [Character] = []

    for char in input.characters {
        if let correspondingBracket = brackets[char] {
            stack.append(correspondingBracket)
        } else if stack.last == char {
            stack.popLast()
        } else if brackets.values.contains(char) {
            return false
        }
    }

    return stack.count == 0
}