13. Roman to Integer

題目原文

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.

解題思路

1. 小的數字在大的數字之前出現，數值等於後面減掉前面。

2. 大的數字在小的數字之前出現或是前後是相同的數字，數值等於兩兩相加。

程式解答

class Solution 
{
public:
    int romanToInt(string s) 
    {
        int answer = 0;
        std::map<char, int> p;
        std::map<char, int>::iterator it;
        
        p['I'] = 1;
        p['V'] = 5;
        p['X'] = 10;
        p['L'] = 50;
        p['C'] = 100;
        p['D'] = 500;
        p['M'] = 1000;
        
        for (int i = 0; i < s.length(); )
        {
            if (p[s[i]] < p[s[i+ 1]])
            {
                answer += p[s[i + 1]] - p[s[i]];
                i += 2;
            }
            else
            {
                answer += p[s[i]];
                i++;
            }
        }
        return answer;
    }
};