# [13. Roman to Integer (Easy)](https://leetcode.com/problems/roman-to-integer/)
<p>Roman numerals are represented by seven different symbols: <code>I</code>, <code>V</code>, <code>X</code>, <code>L</code>, <code>C</code>, <code>D</code> and <code>M</code>.</p>
<pre><strong>Symbol</strong> <strong>Value</strong>
I 1
V 5
X 10
L 50
C 100
D 500
M 1000</pre>
<p>For example, <code>2</code> is written as <code>II</code> in Roman numeral, just two one's added together. <code>12</code> is written as <code>XII</code>, which is simply <code>X + II</code>. The number <code>27</code> is written as <code>XXVII</code>, which is <code>XX + V + II</code>.</p>
<p>Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not <code>IIII</code>. Instead, the number four is written as <code>IV</code>. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as <code>IX</code>. There are six instances where subtraction is used:</p>
<ul>
<li><code>I</code> can be placed before <code>V</code> (5) and <code>X</code> (10) to make 4 and 9. </li>
<li><code>X</code> can be placed before <code>L</code> (50) and <code>C</code> (100) to make 40 and 90. </li>
<li><code>C</code> can be placed before <code>D</code> (500) and <code>M</code> (1000) to make 400 and 900.</li>
</ul>
<p>Given a roman numeral, convert it to an integer.</p>
<p> </p>
<p><strong>Example 1:</strong></p>
<pre><strong>Input:</strong> s = "III"
<strong>Output:</strong> 3
</pre>
<p><strong>Example 2:</strong></p>
<pre><strong>Input:</strong> s = "IV"
<strong>Output:</strong> 4
</pre>
<p><strong>Example 3:</strong></p>
<pre><strong>Input:</strong> s = "IX"
<strong>Output:</strong> 9
</pre>
<p><strong>Example 4:</strong></p>
<pre><strong>Input:</strong> s = "LVIII"
<strong>Output:</strong> 58
<strong>Explanation:</strong> L = 50, V= 5, III = 3.
</pre>
<p><strong>Example 5:</strong></p>
<pre><strong>Input:</strong> s = "MCMXCIV"
<strong>Output:</strong> 1994
<strong>Explanation:</strong> M = 1000, CM = 900, XC = 90 and IV = 4.
</pre>
<p> </p>
<p><strong>Constraints:</strong></p>
<ul>
<li><code>1 <= s.length <= 15</code></li>
<li><code>s</code> contains only the characters <code>('I', 'V', 'X', 'L', 'C', 'D', 'M')</code>.</li>
<li>It is <strong>guaranteed</strong> that <code>s</code> is a valid roman numeral in the range <code>[1, 3999]</code>.</li>
</ul>
**Related Topics**:
[Math](https://leetcode.com/tag/math/), [String](https://leetcode.com/tag/string/)
**Similar Questions**:
* [Integer to Roman (Medium)](https://leetcode.com/problems/integer-to-roman/)
## Solution 1.
```cpp
// OJ: https://leetcode.com/problems/roman-to-integer/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
private:
int charToNum(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;
}
return 0;
}
public:
int romanToInt(string s) {
int ans = 0, last = INT_MAX;
for (char c : s) {
int n = charToNum(c);
if (n > last) ans -= 2 * last;
ans += n;
last = n;
}
return ans;
}
};
```
