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

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

> 题目翻译: 对于给定的两个二进制数字所表达的字符串，我们求其相加所得到的结果， 根据上例便可得到答案.

> 题目分析: 我认为这道题所要注意的地方涵盖以下几个方面:

1. 对字符串的操作.
2. 对于加法，我们应该建立一个进位单位，保存进位数值.
3. 我们还要考虑两个字符串如果不同长度会怎样.
4. int 类型和char类型的相互转换.

> 时间复杂度:其实这就是针对两个字符串加起来跑一遍，O(n) n代表长的那串字符串长度.

```c++
class Solution {
public:
string addBinary(string a, string b) {
int len1 = a.size();
int len2 = b.size();
if(len1 == 0)
return b;
if(len2 == 0)
return a;

string ret;
int carry = 0;
int index1 = len1-1;
int index2 = len2-1;

while(index1 >=0 && index2 >= 0)
{
int num = (a[index1]-'0')+(b[index2]-'0')+carry;
carry = num/2;
num = num%2;
index1--;
index2--;
ret.insert(ret.begin(),num+'0');
}

if(index1 < 0&& index2 < 0)
{
if(carry == 1)
{
ret.insert(ret.begin(),carry+'0');
return ret;
}
}

while(index1 >= 0)
{
int num = (a[index1]-'0')+carry;
carry = num/2;
num = num%2;
index1--;
ret.insert(ret.begin(),num+'0');
}
while(index2 >= 0)
{
int num = (b[index2]-'0')+carry;
carry = num/2;
num = num%2;
index2--;
ret.insert(ret.begin(),num+'0');
}
if(carry == 1)
ret.insert(ret.begin(),carry+'0');
return ret;
}
};

```