> For the complete documentation index, see [llms.txt](https://celia-qian.gitbook.io/leetcode-notebook-2020-2021/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://celia-qian.gitbook.io/leetcode-notebook-2020-2021/others/untitled.md).

# Roman number code

## Description

[integer to roman](https://leetcode.com/problems/integer-to-roman/submissions/)

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, `2` is written as `II` in Roman numeral, just two one's 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 an integer, convert it to a roman numeral.

**Example 1:**

```
Input: num = 3
Output: "III"
```

**Example 2:**

```
Input: num = 4
Output: "IV"
```

**Example 3:**

```
Input: num = 9
Output: "IX"
```

**Example 4:**

```
Input: num = 58
Output: "LVIII"
Explanation: L = 50, V = 5, III = 3.
```

**Example 5:**

```
Input: num = 1994
Output: "MCMXCIV"
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.
```

**Constraints:**

* `1 <= num <= 3999`

## Solution

```cpp
class Solution {
public:
string intToRoman(int num) {
       vector<pair<int, string>> table = { {1000, "M"},
                                           {900, "CM"},
                                           {500, "D"},
                                          {400, "CD"},
                                          {100, "C"},
                                          {90, "XC"},
                                          {50, "L"},
                                          {40, "XL"},
                                          {10, "X"},
                                          {9, "IX"},
                                          {5, "V"},
                                          {4, "IV"},
                                          {1, "I"} };
        auto it = table.begin();
        string roman = "";
        while(num > 0 && it != table.end()){
                if(it->first <= num){
                    roman += it->second;
                    num -= it->first;
                }
                else it++;
            }
          return roman;                                 
    }
};
```

## Description

[roman to integer](https://leetcode.com/problems/roman-to-integer/)

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, `2` is written as `II` in Roman numeral, just two one's 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.&#x20;
* `X` can be placed before `L` (50) and `C` (100) to make 40 and 90.&#x20;
* `C` can be placed before `D` (500) and `M` (1000) to make 400 and 900.

Given a roman numeral, convert it to an integer.

**Example 1:**

```
Input: s = "III"
Output: 3
```

**Example 2:**

```
Input: s = "IV"
Output: 4
```

**Example 3:**

```
Input: s = "IX"
Output: 9
```

**Example 4:**

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

**Example 5:**

```
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]`.

## Solution

```cpp
class Solution {
public:
    int romanToInt(string s) {
        unordered_map<char,int>mp={
            {'I',1},
            {'V',5},
            {'X',10},
            {'L',50},
            {'C',100},
            {'D',500},
            {'M',1000}
        };
        
        int n=s.size();
        int ans=0;
        for(int i=0;i<s.size();i++){
            if(i+1<n && mp[s[i]]<mp[s[i+1]]){
                ans+=mp[s[i+1]]-mp[s[i]];
                i++;
            }else{
                ans+=mp[s[i]];
            }
        }
        return ans;
    }
};
```