list的反轉
Collections.sort(list, Collections.reverseOrder());

 1 class Solution {
 2     public String intToRoman(int num) {
 3         HashMap<Integer, String> map = new HashMap<>();
 4         map.put(1, "I");
 5         map.put(4, "IV");
 6         map.put(5, "V");
 7         map.put(9, "IX");
 
 8         map.put(10, "X");
 9         map.put(40, "XL");
10         map.put(50, "L");
11         map.put(90, "XC");
12         map.put(100, "C");
13         map.put(400, "CD");
14         map.put(500, "D");
15         map.put(900, "CM");
16         map.put(1000, "M");
17         
18         StringBuilder sb = new
 
 StringBuilder();
19         List<Integer> integers = new ArrayList<Integer>(map.keySet());
20         Collections.sort(integers, Collections.reverseOrder());
21         for(int integer : integers) {
22             int quotient = num / integer;
23             for(int i = 0; i < quotient; i++) {
 
24                 sb.append(map.get(integer));
25             }
26             num = num % integer;
27         }
28         
29         return sb.toString();
30     }
31 }

12. Integer to Roman