学步园

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;
    }
};