Add Binary

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

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

class Solution {
public:
    string addBinary(string a, string b) {
        int aSize = a.size();
        int bSize = b.size();
        int len = aSize>bSize?aSize+1:bSize+1;
        string res(len,'0');
        int k = len-1;
        int c = 0;
        int i=aSize-1,j=bSize-1;
        for(;i>=0 && j>=0;i--,j--){
            int tmp = ((a[i]-'0') + (b[j]-'0') + c )%2;
            c  = ((a[i]-'0') + (b[j]-'0') + c )/2;
            res[k--] = tmp+'0';
        }
        while(i>=0){
            int tmp = ((a[i]-'0') + c )%2;
            c  = ((a[i]-'0') + c )/2;
            res[k--] = tmp+'0';
            i--;
        }
        while(j>=0){
            int tmp = ((b[j]-'0') + c )%2;
            c  = ((b[j]-'0') + c )/2;
            res[k--] = tmp+'0';
            j--;
        }
        if(c!=0){
            res[k] = c+'0';
        }else{
            res.erase(res.begin(),res.begin()+k+1);
        }
        
        return res;
    }
};