43. Multiply Strings 字符串表示的大数乘法

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

string multiply(string& num, char ch){
    int n = ch - '';
    string s;
    int carry = ;
    int x;
    for(int i=num.size()-; i>=; i--){
        x = (num[i]-'') * n + carry;
        carry = x/;
        s.insert(s.begin(), x%+'');
    }
    if (carry>) {
        s.insert(s.begin(), carry+'');
    }
    return s;
}

string strPlus(string& num1, string& num2) {
    string s;
    int carry=;
    int x;
    int n1 = num1.size();
    int n2 = num2.size(); 

    int i, j;
    for(i=n1-, j=n2-; i>= || j>=; i--, j--){
        int x1 = i>= ?  num1[i]-'' : ;
        int x2 = j>= ?  num2[j]-'' : ;
        x = x1 + x2 + carry;
        carry = x/;
        s.insert(s.begin(), x%+'');
    }
    if (carry>) {
        s.insert(s.begin(), carry+'');
    }
    return s;
}

string multiply(string num1, string num2) {

    if (num1.size()<= || num2.size()<=) return "";

    int shift=;
    string result="";
    for (int i=num1.size()-; i>=; i--) {
        string s = multiply(num2, num1[i]);
        for(int j=; j<shift; j++){
            s.insert(s.end(), '');
        }
        result = strPlus(result, s);
        shift++;
    }
    //check if it is zero
    if (result[]=='') return "";
    return result;
}