Leetcode-13- Roman numeral to integer (simple)

13. Roman numeral to integer （ Simple ）

Roman numerals contain the following seven characters : I, V, X, L,C,D and M.

 character            The number 
I             1
V             5
X             10
L             50
C             100
D             500
M             1000

for example , Rome digital 2 Write to do II , Two parallel 1.12 Write to do XII , That is to say X + II . 27 Write to do XXVII, That is to say XX + V + II .

Usually , The small numbers in roman numbers are to the right of the big ones . But there are special cases , for example 4 Do not write IIII, It is IV. Numbers 1 In number 5 Left side , The number represented is equal to the large number 5 Decimal reduction 1 Value obtained 4 . similarly , Numbers 9 Expressed as IX. This special rule only applies to the following six cases ：

• I Can be placed in V (5) and X (10) Left side , To express 4 and 9.
• X Can be placed in L (50) and C (100) Left side , To express 40 and 90.
• C Can be placed in D (500) and M (1000) Left side , To express 400 and 900.

Given a Roman number , Convert it to an integer . Input to ensure 1 To 3999 Within the scope of .

Ideas ：

1. When the string does not exist ’IV\IX\XL\XC\CD\CM’ when , Each character of the string represents a number , And , here , The number represented by the preceding character is not less than the number represented by the following character , Such as LVIII=50+5+1+1+1=58.
2. When... Exists in the string ’IV\IX\XL\XC\CD\CM’ when , As such IV The value represented is V-I=4. therefore , When the number represented by the previous character is less than the next character when , The number represented by the previous character needs to be subtracted .
3. therefore , Traverse Roman numerals , Judge the relationship between the current character and the next character , If the current character is smaller then res Subtract this number , otherwise , Add this number .

（1）C++

class Solution {
public:
    int romanToInt(string s) {
        char a[] = {'M','D','C','L','X','V','I'};
        int b[] = { 1000,500,100,50,10,5,1 };
        int n = s.length();
        int res = 0;
        map<char , int> m;
        for(int i=0; i<7; i++){
            m[a[i]]=b[i];
        }
        for(int j=0;j<n-1;j++){   //C++ No boundary detection in , You need to pay attention to yourself 
            if(m[s[j]]<m[s[j+1]])
                res-=m[s[j]];
            else
                res+=m[s[j]];
        }
        res+=m[s[n-1]];
        return res;
    }
};

（2）python

class Solution:
    def romanToInt(self, s: str) -> int:
        a = {
    "M":1000, "D":500, "C":100, "L":50, "X":10,  "V":5, "I":1}
        temp = 0
        res = 0
        for each in s:
            res += a[each]
            if(a[each]>temp):
                res -= 2*temp
            temp = a[each]
        return res