def add_binary(str1, str2):
    result = []  # To store the binary sum
    carry = 0     # To keep track of the carry during addition
    
    # Initialize pointers to the end of the binary strings
    ptr1 = len(str1) - 1
    ptr2 = len(str2) - 1
    
    # Iterate through the binary strings from right to left
    while ptr1 >= 0 or ptr2 >= 0:
        # Get digits(0 or 1) from strings at pointers positions,
        # if either pointer goes beyond the string’s length, then 0 is used.
        if ptr1 >= 0:
            digit1 = ord(str1[ptr1]) - ord('0')
        else:
            digit1 = 0
        
        if ptr2 >= 0:
            digit2 = ord(str2[ptr2]) - ord('0')
        else:
            digit2 = 0
        
        # Calculate the sum of current digits and carry
        total_sum = digit1 + digit2 + carry
        
        # Calculate the current digit value (0 or 1)
        current_digit = total_sum % 2
        
        # Update the carry for the next iteration
        carry = total_sum // 2
        
        # Add the current digit to the result
        result.append(current_digit)
        
        # Move the pointers to the left
        ptr1 -= 1
        ptr2 -= 1
    
    # If there's still a carry left, add it to the result
    if carry:
        result.append(carry)
    
    # Reverse the result list
    reversed_result = result[::-1]
    
    # Convert each digit in the result list to a string and join them
    binary_sum = ''.join(str(digit) for digit in reversed_result)
    
    return binary_sum


def main():
    str1_list = ["1100", "1010100", "10101", "1111", "10101100110010101"]
    str2_list = ["0011", "0100011", "01010", "11111", "1011001010110010100"]

    for i in range(len(str1_list)):
        print(str(i+1) + ".\tFirst input string:  ",str1_list[i])
        print("\tSecond input string: ",str2_list[i])
        
        print("\tBinary Sum:          ", add_binary(str1_list[i],str2_list[i]))
        print('-'*100);
  
if __name__ == '__main__':
    main()