【12】Integer to Roman

按9 5 4 1一点一点减

string intToRoman(int num) {
    string res="";
    int c=num/1000;
    res+=string(c,'M');
    num-=(c*1000);
    if(num>=900){res+="CM";num-=900;}
    else if(num>=500){res+="D";num-=500;}
    else if(num>=400){res+="CD";num-=400;}
    c=num/100;
    res+=string(c,'C');
    num-=(c*100);
    if(num>=90){res+="XC";num-=90;}
    else if(num>=50){res+="L";num-=50;}
    else if(num>=40){res+="XL";num-=40;}
    c=num/10;
    res+=string(c,'X');
    num-=(c*10);
    if(num>=9){res+="IX";num-=9;}
    else if(num>=5){res+="V";num-=5;}
    else if(num>=4){res+="IV";num-=4;}
    c=num;
    res+=string(c,'I');
    return res;
}