0067.py

class Solution:
    def addBinary(self, a, b):
        """
        :type a: str
        :type b: str
        :rtype: str
        """
        res = list()
        add, l1, l2 = 0, len(a), len(b)
        while add or l1 or l2:
            if l1:
                l1 -= 1
                add += int(a[l1])
            if l2:
                l2 -= 1
                add += int(b[l2])
            res.append(str(add & 1))
            add >>= 1
            
        return ''.join(reversed(res))