class Solution:
    # @return a boolean
    def isValid(self, s):
        if s == '': return True
        stack = []
        left = '([{'; right = ')]}'
        for i in s:
            if i == '(' or i == '[' or i == '{':
                stack.append(i); continue
            for j in xrange(3):
                if i == right[j]:
                    if not stack or stack[-1] != left[j]:
                        return False
                    else:
                        stack.pop(); continue
        return not stack