roman_to_integer.dart

/*

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-* 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 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.



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].


 */

void main() {
  final a = A();
  final ac = a.romanToInt("XXI");
  print(ac);
  final b = B();
  final ab = b.romanToInt("XXI");
  print(ab);
}

class A {
  int romanToInt(String s) {
    // map to keep tract of the symbols
    Map<String, int> romanMap = <String, int>{
      "I": 1,
      "V": 5,
      "X": 10,
      "L": 50,
      "C": 100,
      "D": 500,
      "M": 1000,
    };
    // entire length of the entered string - whatever we will enter
    int n = s.length;

    // variable to store result
    int? nums = romanMap[s[n - 1]];
    // loop for every character fromm right to left
    for (var i = n - 2; i >= 0; i--) {
      // to  check if the right character is bigger than left
      if (romanMap[s[i]]! >= romanMap[s[i + 1]]!) {
        // if it is not big we  will add the value
        if (nums != null) {
          nums += romanMap[s[i]]!;
        }
      } else {
        // if it is not bigger than we will remove the value
        if (nums != null) {
          nums -= romanMap[s[i]]!;
        }
      }
    }
    return nums!;
  }
}

class B {
  int romanToInt(String s) {
    int getInt(String s) {
      switch (s) {
        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:
          -1;
      }
      return 0;
    }

    int n = s.length;
    int result = 0;
    int current = 0;
    int next = 0;
    int i = 0;

    while (i < n) {
      if (i == n - 1) {
        result += getInt(s[i]);
        return result;
      }
      current = getInt(s[i]);
      next = getInt(s[i + 1]);
      if (current >= next) {
        result += current;
        i++;
      } else {
        result += next - current;
        i += 2;
      }
    }

    return result;
  }
}