Add Binary Strings

add_binary.py

"""
Description:
Given two binary strings, return their sum (also a binary string).
The input strings are both non-empty and contains only characters 1 or 0.

Example:
Input: a = "1010", b = "1011"
Output: "10101"
"""

# much faster using built-in function
def addBinary(a, b):
    """
    :type a: str
    :type b: str
    :rtype: str
    """
    return '{:b}'.format(int(a, 2) + int(b, 2))

# slower but doesn't use Python specific built-in function
def addBinary(a, b):
    """
    :type a: str
    :type b: str
    :rtype: str
    """
    result = ''
    idx_a, idx_b = len(a) - 1, len(b) - 1
    carry = 0
    while idx_a >= 0 or idx_b >= 0:
        d_a = 1 if idx_a >= 0 and a[idx_a] == '1' else 0
        d_b = 1 if idx_b >= 0 and b[idx_b] == '1' else 0
        tmp = carry + d_a + d_b
        carry, rem = tmp // 2, tmp % 2
        result = str(rem) + result
        idx_a -= 1
        idx_b -= 1
    if carry == 1:
        result = '1' + result
    return result