13. Roman to Integer，罗马数字到整数

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M. （罗马数字都这么小吗？）

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

For example, 2 is written as II in Roman numeral, just two ones added together. 12 is written as XII, which is simply X + II. The number 27 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. 输入一个罗马数字，转换成int。

Example 1:
Input: s = "III"
Output: 3
Explanation: III = 3.

Example 2:
Input: s = "LVIII"
Output: 58
Explanation: L = 50, V= 5, III = 3.

Example 3:
Input: s = "MCMXCIV"
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

Constraints:

1 <= s.length <= 15
s contains only the characters ('I', 'V', 'X', 'L', 'C', 'D', 'M').
It is guaranteed that s is a valid roman numeral in the range [1, 3999].

Python

RT = {'I':1,'V':5,'X':10,'L':50,'C':100,'D':500,'M':1000}

class Solution:
    def romanToInt(self, s: str) -> int:
        rtv = last = 0
        for c in reversed(s):
            if RT[c] < last:
                rtv -= RT[c]
            else:
                rtv += RT[c]
            last = RT[c]
        return rtv

C++

static unordered_map<char,int> RT{{'I',1},
                                  {'V',5},
                                  {'X',10},
                                  {'L',50},
                                  {'C',100},
                                  {'D',500},
                                  {'M',1000}};


class Solution {
public:
    int romanToInt(string s) {
        int rtv{}, last{};
        for(auto i{s.rbegin()}; i<s.rend(); ++i){
            int v{RT[*i]};
            if(v < last)
                rtv -= v;
            else
                rtv += v;
            last = v;
        }
        return rtv;
    }
};

static int toV(char c){
    switch(c){
        case 'I':
            return 1;
        case 'V':
            return 5;
        case 'X':
            return 10;
        case 'L':
            return 50;
        case 'C':
            return 100;
        case 'D':
            return 500;
        case 'M':
            return 1000;
        default:
            return 0;
    }
}


int romanToInt(char *s){
    int rtv = 0;
    int last = 0;
    for(int i=strlen(s)-1; i>=0; --i){
        int v = toV(s[i]);
        if(v < last)
            rtv -= v;
        else
            rtv += v;
        last = v;
    }
    return rtv;
}

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13. Roman to Integer，罗马数字到整数

题目

Python

C++

C