[LeetCode]67. Add Binary

class Solution:
    def addBinary(self, a: str, b: str) -> str:
        i, j, carry = len(a) - 1, len(b) - 1, 0
        result = []
        while i >= 0 and j >= 0:
            digit = (int(a[i]) + int(b[j]) + carry) % 2
            carry = (int(a[i]) + int(b[j]) + carry) // 2
            result.insert(0, digit)
            i -= 1
            j -= 1
        while i >= 0:
            digit = (int(a[i]) + carry) % 2
            carry = (int(a[i]) + carry) // 2 
            result.insert(0, digit)
            i -= 1
            j -= 1
        while j >= 0:
            digit = (int(b[j]) + carry) % 2
            carry = (int(b[j]) + carry) // 2 
            result.insert(0, digit)
            i -= 1
            j -= 1
        if carry == 1:
            result.insert(0, carry)
        
        return ‘‘.join(map(str, result))