func longestValidParentheses(s string) int {
    var stack []int
    var lastStart, maxLen int
    for i, item := range s {
        if string(item) == "(" {
            stack = append(stack, i)
        } else {
            if len(stack) > 0 {
                stack = stack[:len(stack) - 1]
                var length int
                if len(stack) == 0 {
                    length = i - lastStart + 1
                } else {
                    length = i - stack[len(stack) - 1]
                }
                if length > maxLen {
                    maxLen = length
                }
            } else {
                lastStart = i + 1
            }
        }
    }
    return maxLen
}