# 13. Roman to Integer (Easy)

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, 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.```

## Solutions

### 1.

``````class Solution {

public int romanToInt(String s) {
int ans = 0;

if (s == null || s.isEmpty()) {
return ans;
}

// Build the map between the symbols and corresponding values
int[] nums = {1, 4, 5, 9, 10, 40, 50, 90, 100, 400, 500, 900, 1000};
String[] roman = {"I", "IV", "V", "IX", "X", "XL", "L", "XC", "C", "CD", "D", "CM", "M"};

while (!s.isEmpty()) {
String rn = "";
for (int i = roman.length - 1; i >= 0; i--) {
if (s.startsWith(roman[i])) {
rn = roman[i];
ans += nums[i];
break;
}
}

// rn can not be empty
s = s.substring(rn.length());
}

return ans;
}
}
``````

### 2.

``````class Solution {

public int romanToInt(String s) {
int ans = 0;

if (s == null || s.isEmpty()) {
return ans;
}

// Build the map between the symbols and corresponding values
Map<Character, Integer> map = new HashMap<>();

map.put('I', 1);
map.put('V', 5);
map.put('X', 10);
map.put('L', 50);
map.put('C', 100);
map.put('D', 500);
map.put('M', 1000);

int pre = 0;

// XLVI=46, L=50, X=10, V=5, I=1
for (int i = 0; i < s.length(); i++) {
Character c = s.charAt(i);
int val = map.get(c);

// set the previous value for the reason that there is no previous value at first position.
if (i == 0) {
pre = val;
continue;
}

// minus it if previous value is less than current, such as XL, X is -10
if (pre < val) {
ans -= pre;
}
// plus it once previous value is greater than current, for example LX, X is 10
else {
ans += pre;
}

pre = val;
}

// The last position value is not added. The fact is that the last value is always a positive one.
return ans + pre;
}
}
``````