将数字转换为罗马数字（Integer to Roman）

https://leetcode.com/problems/integer-to-roman/description/

题目大意：将数字转化为罗马数字（数字范围1--3999）

思路：罗马数字的构成

          1(I)   5(V)   10(X)  50(L)   100(C)   500(D)    1000(M)

          以上重复几次就是几倍，如20（XX）

          不过最多只能重复3次，因此有5，50，500的表示 。小大是减如40=50-10（XL）    大小是加60=50+10(LX)

          因此只要会表示出个级，十级，百级，千级，一个数就可以表示了

class Solution {
public:
string intToRoman(int num) {
string M[]={"","M","MM","MMM"};
string C[]={"","C","CC","CCC","CD","D","DC","DCC","DCCC","CM"};
string X[]={"","X","XX","XXX","XL","L","LX","LXX","LXXX","XC"};
string I[]={"","I","II","III","IV","V","VI","VII","VIII","IX"};
return M[num/1000]+C[(num%1000)/100]+X[(num%100)/10]+I[(num%10)];
}
};