Leetcode: Integer to Roman

 1 public class Solution {
 2     public String intToRoman(int num) {
 3         StringBuffer res = new StringBuffer();
 4         if (num > 3999 || num < 1) return res.toString();
 5         int copy = num;
 6         int thousand = copy / 1000; //# of thousand
 7         copy = copy - thousand * 1000;
 8         int hundred = copy / 100; //# of hundred
 9         copy = copy - hundred * 100;
10         int ten = copy / 10; //# of ten
11         copy = copy - ten * 10;
12         int one = copy / 1; //# of one
13         
14         if (thousand != 0) { //thousand
15             switch (thousand) {
16                 case 1: res.append("M"); break;
17                 case 2: res.append("MM"); break;
18                 case 3: res.append("MMM"); break;
19             }
20         }
21         
22         if (hundred != 0) { //thousand
23             switch (hundred) {
24                 case 1: res.append("C"); break;
25                 case 2: res.append("CC"); break;
26                 case 3: res.append("CCC"); break;
27                 case 4: res.append("CD"); break;
28                 case 5: res.append("D"); break;
29                 case 6: res.append("DC"); break;
30                 case 7: res.append("DCC"); break;
31                 case 8: res.append("DCCC"); break;
32                 case 9: res.append("CM"); break;
33             }
34         }
35         
36         if (ten != 0) { //thousand
37             switch (ten) {
38                 case 1: res.append("X"); break;
39                 case 2: res.append("XX"); break;
40                 case 3: res.append("XXX"); break;
41                 case 4: res.append("XL"); break;
42                 case 5: res.append("L"); break;
43                 case 6: res.append("LX"); break;
44                 case 7: res.append("LXX"); break;
45                 case 8: res.append("LXXX"); break;
46                 case 9: res.append("XC"); break;
47             }
48         }
49         
50         if (one != 0) { //thousand
51             switch (one) {
52                 case 1: res.append("I"); break;
53                 case 2: res.append("II"); break;
54                 case 3: res.append("III"); break;
55                 case 4: res.append("IV"); break;
56                 case 5: res.append("V"); break;
57                 case 6: res.append("VI"); break;
58                 case 7: res.append("VII"); break;
59                 case 8: res.append("VIII"); break;
60                 case 9: res.append("IX"); break;
61             }
62         }
63         return res.toString();
64     }
65 }