67. Add Binary. Given two binary strings a and b, return their sum as a binary string.

sum_binary.py

def sum_binary(self, a: str, b: str) -> str:
    # sort pythonic version
    # return str(bin(int(a, 2) + int(b, 2)))[2:]
        
        
    # long imperative version
    if len(a) == 1 and a[0] == '0' and len(b) == 1 and b[0] == '0':
        return '0'
        
    m_len = max(len(a), len(b))
    a = a.zfill(m_len)[::-1]
    b = b.zfill(m_len)[::-1]
    a = [int(num) for num in a]
    b = [int(num) for num in b]
    r = [0] * (len(a) + len(b))

    for i in range(m_len):
        if r[i] == 0:
            if a[i] == 1 and b[i] == 1:
                r[i] = 0
                r[i + 1] = 1
            elif (a[i] == 1 and b[i] == 0) or (a[i] == 0 and b[i] == 1):
                r[i] = 1
        elif r[i] == 1:
            if a[i] == 1 and b[i] == 1:
                r[i] = 1
                r[i + 1] = 1
            elif (a[i] == 1 and b[i] == 0) or (a[i] == 0 and b[i] == 1):
                r[i] = 0
                r[i + 1] = 1

    # remove zeros
    k = 0
    r = r[::-1]
    while r[k] == 0:
        k += 1
    r = r[k:]
    r = [str(num) for num in r]
    result = ''.join(r) 
    return result
    
    
res = sum_binary('111', '11')
print(res)  # 1010

res = sum_binary('11', '1')
print(res)  # 100

res = sum_binary('1010', '1011')
print(res)  # 10101

res = sum_binary('0', '0')
print(res)  # 0