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

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

思路

注意考虑几个条件即可：a和b的长度不同，则位数短的字符串首先停止迭代；注意循环过程中和循环结束后的进位。

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class Solution(object):
    def addBinary(self, a, b):
        """
        :type a: str
        :type b: str
        :rtype: str
        """
        lena = len(a)
        lenb = len(b)
        count = summ = carry = 0
        result = ''
        while (count < lena or count < lenb):
            if (count < lena and count < lenb):
                summ = int(a[-count-1]) + int(b[-count-1]) + carry
            else:
                if (lena > lenb):
                    summ = int(a[-count-1]) + carry
                else:
                    summ = int(b[-count-1]) + carry
            
            if (summ == 2):
                carry = 1
                summ = 0
            elif (summ == 3):
                carry = 1
                summ = 1
            else:
                carry = 0
                
            result = str(summ) + result
            count += 1
        if (carry == 1):
            result = str(carry) + result
        return result