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：
把每一个字母代表的数字存在hashmap里，从第一个字符开始读字符串，如果这个字符比下一个字符代表的数字小，说明需要减去这个字符。如果这个字符比下一个字符代表的数字大，说明要加上这个字符。
Java语言：
public int romanToInt(String s) {
int res = 0;
Map<Character, Integer> map = new HashMap<>();
map.put('I', 1);
map.put('V', 5);
map.put('X', 10);
map.put('L', 50);
map.put('C', 100);
map.put('D', 500);
map.put('M', 1000);
for (int i = 0; i < s.length(); i++) {
if (i != s.length() - 1 && map.get(s.charAt(i)) < map.get(s.charAt(i + 1))) {
res -= map.get(s.charAt(i));
res += map.get(s.charAt(i + 1));
i++;
} else {
res += map.get(s.charAt(i));
}
}
return res;
}
