Strobogrammatic Number — Python

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

class Solution(object):
    lookup = {'0':'0', '1':'1', '6':'9', '8':'8', '9':'6'}

    # @param {string} num
    # @return {boolean}
    def isStrobogrammatic(self, num):
        n = len(num)
        for i in xrange((n+1) / 2):
            if num[n-1-i] not in self.lookup or \
               num[i] != self.lookup[num[n-1-i]]:
                return False
        return True