给定两个二进制字符串，返回他们的和（用二进制表示）。
输入为非空字符串且只包含数字 1 和 0。
class Solution {
public:
string addBinary(string a, string b) {
string s;
int k = 0;
int n = (a.size()<=b.size())?a.size():b.size();
string sum = (a.size()>=b.size())?a:b;
for(int i = 1; i <= n; ++i)
{
if(k==0)
{
if(a[a.size()-i]=='0')
{
if(b[b.size()-i]=='0')
{
s.push_back('0');
k=0;
}
else
{
s.push_back('1');
k=0;
}
}
else
{
if(b[b.size()-i]=='0')
{
s.push_back('1');
k=0;
}
else
{
s.push_back('0');
k=1;
}
}
}
else
{
if(a[a.size()-i]=='0')
{
if(b[b.size()-i]=='0')
{
s.push_back('1');
k=0;
}
else
{
s.push_back('0');
k=1;
}
}
else
{
if(b[b.size()-i]=='0')
{
s.push_back('0');
k=1;
}
else
{
s.push_back('1');
k=1;
}
}
}
}
for(int i=n+1; i<=sum.size(); ++i)
{
if(k==0)
{
s.push_back(sum[sum.size()-i]);
}
else
{
if(sum[sum.size()-i]=='0')
{
s.push_back('1');
k=0;
}
else
{
s.push_back('0');
k=1;
}
}
}
if(k==1)
{
s.push_back('1');
}
reverse(s.begin(), s.end());
return s;
}
};