20.有效的括号

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

func isValid(_ s: String) -> Bool {
    if s.count <= 0 {return true}
    var result = [Character]()
    for (_, char) in s.enumerated() {
        if char == "(" || char == "{" || char == "[" {
            result.append(char)
        } else {
            if result.isEmpty {return false}
            //获取result最后一个char
            let lastChar = result.popLast()
            if lastChar == "(" && char != ")"{return false}
            if lastChar == "[" && char != "]"{return false}
            if lastChar == "{" && char != "}"{return false}
        }
    }
    return result.isEmpty
}