13. Roman to Integer(罗马数字转换成数字)（leetcode）

题目：

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

Example 1:

Input: "III"
Output: 3

Example 2:

Input: "IV"
Output: 4

Example 3:

Input: "IX"
Output: 9

Example 4:

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

Example 5:

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

分析：本题是把罗马数字转换成数字。难点在于出现IV时如何转换。

1、之前我是用if - else语句写的。代码多不清晰。遇到可以组成键值对的一定要想到hashmap集合。

2、把字符串转换成数组的方法：String 类中public char charAt(int index)

　　　　　　　　　　　　　　　                public char[] toCharArray()

　　　　　　　　　　　　　　　　　　　public String[] split(String regex)

把数组转换成字符串的方法：构造方法 String str = new String(data);

　　　　　　　　　　　　　String.valueOf(char[] ch)