leetcode 日经贴，python code -Valid Parentheses

Valid Parentheses

class Solution:
    # @return a boolean
    def isValid(self, s):
        st = []
        for x in s:
            if x in ('(', '[', '{'):
                st.append(x)
            elif len(st) > 0:
                if x == ')' and st[-1] == '(' or \
                    x == ']' and st[-1] == '[' or \
                    x == '}' and st[-1] == '{':
                    st.pop()
                else:
                    return False
            else:
                return False
        return False if st else True