func isValid(s1 string) bool {
    stack := []rune{}

    for _, c := range s1 {
        switch c {
        case '(':
            stack = append(stack, ')')
        case '[':
            stack = append(stack, ']')
        case '{':
            stack = append(stack, '}')
        case ')', ']', '}':
            if len(stack) == 0 || stack[len(stack) - 1] != c {
                return false
            }
            
            // remove the last element from the stack
	          stack = stack[:len(stack) - 1]
        }
    }

    return len(stack) == 0
}