12. Integer to Roman

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.

Symbol       Value
I             1
V             5
X             10
L             50
C             100
D             500
M             1000

For example, two is written as II in Roman numeral, just two one's added together. Twelve is written as, XII, which is simply X + II. The number twenty seven is written as XXVII, which is XX + V + II.

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, the number four is written as IV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used:

• I can be placed before V (5) and X (10) to make 4 and 9.
• X can be placed before L (50) and C (100) to make 40 and 90.
• C can be placed before D (500) and M (1000) to make 400 and 900.

Given an integer, convert it to a roman numeral. Input is guaranteed to be within the range from 1 to 3999.

Example 1:
Input: 3
Output: "III"

Example 2:
Input: 4
Output: "IV"

Example 3:
Input: 9
Output: "IX"

Example 4:
Input: 58
Output: "LVIII"
Explanation: L = 50, V = 5, III = 3.

Example 5:
Input: 1994
Output: "MCMXCIV"
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

难度：medium

题目：
罗马数字由七个符号组成：I, V, X, L, C, D 和 M.
例如，2的罗马数为II，两个I相挨。然而4并不是IIII.而是IV. 因为1是在5前所以由5减1构成4。同样的规则适合9罗马数为IX. 有6个实例需要用这种减规则：

• I 可以放置在V 和 X之前构成4和9
• X 可以放置在L 和 C之前构成40和90
• C 可以放置在D 和 M之前构成400和900

Runtime: 35 ms, faster than 96.86% of Java online submissions for Integer to Roman.

class Solution {
    public String intToRoman(int num) {
        StringBuilder sb = new StringBuilder();
        for (; num > 0;) {
            if (num - 1000 >= 0) {
                sb.append("M");
                num -= 1000;
            } else if (num - 900 >= 0) {
                sb.append("CM");
                num -= 900;
            } else if (num - 500 >= 0) {
                sb.append("D");
                num -= 500;
            } else if (num - 400 >= 0) {
                sb.append("CD");
                num -= 400;
            } else if (num - 100 >= 0) {
                sb.append("C");
                num -= 100;
            } else if (num - 90 >= 0) {
                sb.append("XC");
                num -= 90;
            } else if (num - 50 >= 0) {
                sb.append("L");
                num -= 50;
            } else if (num - 40 >= 0) {
                sb.append("XL");
                num -= 40;
            }  else if (num - 10 >= 0) {
                sb.append("X");
                num -= 10;
            } else if (num - 9 >= 0) {
                sb.append("IX");
                num -= 9;
            } else if (num - 5 >= 0) {
                sb.append("V");
                num -= 5;
            } else if (num - 4 >= 0) {
                sb.append("IV");
                num -= 4;
            } else if (num - 1 >= 0) {
                sb.append("I");
                num -= 1;
            }
        }
        
        return sb.toString();
    }
}