6.26 ->30 addBinary & lon

• to do

1] Add Binary
表示数字时记住转换char<=>int

    string addBinary(string a, string b) {
        int carry = 0;
        string ret = "";
        for (auto i=a.rbegin(), j=b.rbegin(); i<a.rend() || j<b.rend(); ) {
            int n1 = i<a.rend()? *i++ -'0' : 0;
            int n2 = j<b.rend()? *j++ -'0' : 0;
            int sum = n1 + n2 + carry;
            ret.push_back(sum%2 + '0');
            carry = sum>1? 1: 0;
        }
        if (carry) ret.push_back(carry+'0');
        reverse(ret.begin(), ret.end());
        return ret;
    }

5] Longest Common Prefix

note the if statement for bound-checking. Although in c++11, a null terminator is guaranteed to guard anyways.

    string longestCommonPrefix(vector<string>& strs) {
        if (strs.empty()) return "";
        for (int i=0; i<strs[0].size(); ++i) {
            for (int si=1; si<strs.size(); ++si) {
                if (i>=strs[si].size() || strs[si][i]!=strs[0][i]) //note!
                    return strs[0].substr(0,i);
            }
        }
        return strs[0];
    }