【LeetCode】13. Roman to Integer - Java实现

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1. 题目描述：

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

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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.

2. 思路分析：

题目的意思是将罗马数字转成普通int，并且题目给出了罗马数字的表示方式。

思路就是遍历罗马字母，如果当前的罗马字母表示的数字比右边的大，则直接累加即可，如果比右边的小，则需要减去当前罗马字母表示的数字。

3. Java代码：

源代码：见我GiHub主页

代码：

public static int romanToInt(String s) {
    Map<Character, Integer> romanMap = new HashMap<>();
    romanMap.put('I', 1);
    romanMap.put('V', 5);
    romanMap.put('X', 10);
    romanMap.put('L', 50);
    romanMap.put('C', 100);
    romanMap.put('D', 500);
    romanMap.put('M', 1000);
    int result = 0;
    for (int i = 0; i < s.length(); i++) {
        if (i == s.length() - 1 || romanMap.get(s.charAt(i)) >= romanMap.get(s.charAt(i + 1))) {
            result += romanMap.get(s.charAt(i));
        } else {
            result -= romanMap.get(s.charAt(i));
        }
    }
    return result;
}