67.md

67. Add Binary

Given two binary strings, return their sum (also a binary string).

For example,
a = "11"
b = "1"
Return "100".

my thoughts:

1. just follow the rules for binary addition.

my solution:

**********42ms
class Solution(object):
    def addBinary(self, a, b):
        """
        :type a: str
        :type b: str
        :rtype: str
        """
        if len(a) > len(b):
            b, a = a, b
        
        la = len(a)
        lb = len(b)
        prea = '0'*(lb-la)
        a = prea + a
        i = lb-1
        c = 0
        res = []
        while i>=0:
            if a[i] != b[i]:
                n = c+1
                c = n/2
                res.append(str(n%2))
            elif a[i] == '0':
                n = c
                c = 0
                res.append(str(n))
            else:
                n = c+2
                c = 1
                res.append(str(n%2))
            i -= 1
        res.reverse()
        r = ''.join(x for x in res)
        return r if c==0 else '1'+r

my comments:

from other ppl's solution:

1. bin()
bin(x)
Convert an integer number to a binary string. The result is a valid Python expression. If x is not a Python int object, it has to define an __index__() method that returns an integer.

number = 5
print('The binary equivalent of 5 is:', bin(number))

The binary equivalent of 5 is: 0b101
The prefix 0b represents that the result is a binary string.

use all built in funcs, 29ms:
class Solution(object):
    def addBinary(self, a, b):
        """
        :type a: str
        :type b: str
        :rtype: str
        """
        num = int(a,2) + int(b,2)
        return bin(num)[2:]