效率还行 

class Solution {
	public static String intToRoman(int num) {
		if(num < 0 || num>3999) return null;
		char[] romaA = {' ','I','V','X','L','C','D','M'};
					 //{	 1 , 5 , 10, 50,100,500,1000}
		String[] romaB= {" ","XL","XC","CD","CM"};
					//  {	  40 , 90 , 400, 900  }
		//I可以放在V或X的左边，来表示4和9
		//X可以放在L和C的左边表示40和90
		//C可以放在D和M的左边，表示400和900
		//输入在1-3999的范围内
		StringBuilder strbu= new StringBuilder(10);
		int remainder = 0;
		int sign = 1;
		while(num!=0) {
			remainder = num % 10;
			num = num / 10;
			if(sign==1) {
					strbu.append(getRoma(remainder));
			}
			else if(sign==2) {			
				if(remainder<=3) {
					strbu.insert(0, getX(remainder));
				}else if(remainder==4) {
					strbu.insert(0, romaB[1]);
				}else if(remainder==5) {
					strbu.insert(0, romaA[4]); 
				}else if(remainder<9) {
					strbu.insert(0, "L"+getX(remainder-5)); 
				}else if(remainder==9) {
					strbu.insert(0, romaB[2]); 
				} 
			}
			else if(sign==3) {
				if(remainder<=3) {
					strbu.insert(0, getC(remainder));
				}else if(remainder==4) {
					strbu.insert(0, romaB[3]);
				}else if(remainder==5) {
					strbu.insert(0, romaA[6]); 
				}else if(remainder<9) {
					strbu.insert(0, romaA[6]+getC(remainder-5)); 
				}else if(remainder==9) {
					strbu.insert(0, romaB[4]); 
				} 
			}
			else if(sign==4) {
				strbu.insert(0, getM(remainder));
			}
			sign++;
		}
		
		//用例 3999MMMCMXCIX 444CDXLIV
		return strbu.toString();
    }
	public static String getRoma(int num) {
		String[] roma = {"","I","II","III","IV","V","VI","VII","VIII","IX","X"};		
		return roma[num];
	}
	public static String getX(int num) {
		String info = "";
		while(num!=0) {
			info+="X";
			num--;
		}
		return info;
	}
	public static String getC(int num) {
		String info = "";
		while(num!=0) {
			info+="C";
			num--;
		}
		return info;
	}
	public static String getM(int num) {
		String info = "";
		while(num!=0) {
			info+="M";
			num--;
		}
		return info;
	}
}