Add Binary

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

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

不难，考细节

class Solution {public:    string addBinary(string a, string b) {        int len1 = a.size(), len2 = b.size();        string c;        int up = 0;        char a1, b1;        while(len1 > 0 || len2 > 0){          if(len1 <= 0) a1 = '0';          else a1 = a[len1-1];          if(len2 <= 0) b1 = '0';          else b1 = b[len2-1];          int tmp = a1 - '0' + b1 - '0'+ up;          up = 0;          if(tmp == 2 || tmp == 3)             up = 1;          if(tmp %2 == 1)             c = '1' + c;          else             c = '0' + c;             len1--;            len2--;        }        if(up == 1) c = '1' + c;        return c;    }};