No67. Add Binary（Python）

題目：

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 1:

Input: a = "11", b = "1"
Output: "100"

Example 2:

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

思路：

        二進制加法思想就是：從最低位兩位數相加，到2後進一位加到最高項。這就是代碼的整體思路。

代碼：

class Solution(object):
    def addBinary(self, a, b):
        """
        :type a: str
        :type b: str
        :rtype: str
        """
        aIndex,bIndex,flag,s = len(a)-1,len(b)-1,0,''
        while aIndex >= 0 and bIndex >= 0:
            num = int(a[aIndex])+int(b[bIndex])+flag
            flag = num/2
            num %= 2
            s = str(num)+s
            aIndex -= 1
            bIndex -= 1
        while aIndex >= 0:
            num = int(a[aIndex])+flag
            flag = num/2
            num %= 2
            s = str(num)+s
            aIndex -= 1
        while bIndex >= 0:
            num = int(b[bIndex])+flag
            flag = num/2
            num %= 2
            s = str(num)+s
            bIndex -= 1
        if flag == 1:
            s = '1'+ s
        return s