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

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

對string瞭解不熟練，本來想著一直在res前加字元，但不會用，後來就在res後加字元然後呼叫reverse翻轉。其實用:res = (char)(tmp+'0') + res;即可。

class Solution {
public:
    string addBinary(string a, string b) {
        int alen = a.size();
        int blen = b.size();
        if(!alen)
            return b;
        if(!blen)
            return a;

        int carry  = 0;
        string res;
        int i,j;
        //從後向前依次求和
        for(i = alen-1,j = blen-1;i>=0 && j>=0;i--,j--){
            int tmp = a[i]-'0'+b[j]-'0'+carry;
            if(tmp >= 2)
                carry = 1;
            else
                carry = 0;
            res += tmp%2+'0';
        }
        //如果j沒遍歷完，就接著遍歷
        while(j>=0){
            int tmp = b[j]-'0'+carry;
            if(tmp >= 2)
                carry = 1;
            else
                carry = 0;
            res += tmp%2+'0';
            j--;
        }

        while(i>=0){
            int tmp = a[i]-'0'+carry;
            if(tmp >= 2)
                carry = 1;
            else
                carry = 0;
            res += tmp%2+'0';
            i--;
        }
        //處理最後還有進位的情況
        if(carry)
            res+='1';
        reverse(res.begin(),res.end());
        return res;
    }
};