Score of Parentheses — Python

# Time:  O(n)
# Space: O(1)


class Solution(object):
    def scoreOfParentheses(self, S):
        """
        :type S: str
        :rtype: int
        """
        result, depth = 0, 0
        for i in xrange(len(S)):
            if S[i] == '(':
                depth += 1
            else:
                depth -= 1
                if S[i-1] == '(':
                    result += 2**depth
        return result


# Time:  O(n)
# Space: O(h)
class Solution2(object):
    def scoreOfParentheses(self, S):
        """
        :type S: str
        :rtype: int
        """
        stack = [0]
        for c in S:
            if c == '(':
                stack.append(0)
            else:
                last = stack.pop()
                stack[-1] += max(1, 2*last)
        return stack[0]