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#!/usr/bin/env python

'''

Leetcode: Add binary

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

For example,

a = "11"

b = "1"

Return "100".

'''

from __future__ import division

import random

def valid_binary_str(s):

for bit in list(s):

if bit != '0' and bit != '1': return False

return True

# Special cases: negative numbers? not a valid binary string?

def add_binary(a, b):

if (not valid_binary_str(a)) or (not valid_binary_str(b)): return None

print a,'+',b,'=',

# make a and b of equal lengths

l = max(len(a), len(b))

while len(a) < l: a = '0'+a

while len(b) < l: b = '0'+b

i = l-1

results = ""

prev_carry = 0

while i >= 0:

x, y = int(a[i]), int(b[i])

# add current bits

z = x ^ y

cur_carry = x & y

# add previous carry

if prev_carry == 1:

if z == 1: z = 0; cur_carry = 1

else: z = 1

cur_carry = (z & prev_carry) | cur_carry

z = z ^ prev_carry

results = str(z) + results

prev_carry = cur_carry

i -= 1

# last carry

if prev_carry: results = '1'+results

print results

return results

if __name__ == '__main__':

add_binary('11','1')

add_binary('101011','1111')

add_binary('101','1111')

add_binary('0','0')

add_binary('11','1222')