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Largest Roman Numeral possible by rearranging characters of a given string
  • Last Updated : 26 Mar, 2021

Given a string S of length N, the task is to print the highest roman number possible by rearranging the characters of the given string, if the given string is a valid roman number. If found to be not possible, then print “Invalid”.

Examples:

Input: S = “MCMIV”
Output: MMCVI
Explanation: The given string S is valid Roman numeral. Rearranging characters to obtain the string “MMCVI” generates the highest roman numeral possible using given set of characters.

Input: S = “XCYVM”
Output: Invalid

Approach: The idea is to use the sorting by the second element. Follow the steps below to solve the problem:



Below is the implementation of the above approach:

C++




// C++ implementation of the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to find decimal
// value of a roman character
int charVal(char c)
{
    if (c == 'I')
        return 1;
    if (c == 'V')
        return 5;
    if (c == 'X')
        return 10;
    if (c == 'L')
        return 50;
    if (c == 'C')
        return 100;
    if (c == 'D')
        return 500;
    if (c == 'M')
        return 1000;
 
    return -1;
}
 
// Function to convert string representing
// roman number to equivalent decimal value
int romanToDec(string S)
{
    // Stores the decimal value
    // of the string S
    int res = 0;
 
    // Stores the size of the S
    int n = S.size();
 
    // Update res
    res = charVal(S[n - 1]);
 
    // Traverse the string
    for (int i = n - 2; i >= 0; i--) {
 
        if (charVal(S[i]) < charVal(S[i + 1]))
            res -= charVal(S[i]);
        else
            res += charVal(S[i]);
    }
 
    // Return res
    return res;
}
 
// Function to convert decimal
// to equivalent roman numeral
string DecToRoman(int number)
{
    // Stores the string
    string res = "";
 
    // Stores all the digit values of a roman digit
    int num[] = { 1, 4, 5, 9, 10, 40, 50,
                  90, 100, 400, 500, 900, 1000 };
 
    string sym[]
        = { "I", "IV", "V", "IX", "X", "XL", "L",
            "XC", "C", "CD", "D", "CM", "M" };
 
    int i = 12;
 
    // Iterate while number
    // is greater than 0
    while (number > 0) {
 
        int div = number / num[i];
        number = number % num[i];
        while (div--) {
            res += sym[i];
        }
        i--;
    }
 
    // Return res
    return res;
}
 
// Functur to sort the string
// in descending order of values
// assigned to characters
bool compare(char x, char y)
{
    // Return character with
    // highest decimal value
    int val_x = charVal(x);
    int val_y = charVal(y);
 
    return (val_x > val_y);
}
 
// Function to find largest roman
// value possible by rearranging
// the charcaters of the string
string findLargest(string S)
{
    // Stores all roman characters
    set<char> st = { 'I', 'V', 'X', 'L', 'C', 'D', 'M' };
 
    // Traverse the string
    for (auto x : S) {
 
        // If X is not found
        if (st.find(x) == st.end())
            return "Invalid";
    }
    sort(S.begin(), S.end(), compare);
 
    // Stores the decimal value
    // of the roman number
    int N = romanToDec(S);
 
    // Find the roman value equivalent
    // to the decimal value of N
    string R = DecToRoman(N);
 
    if (S != R)
        return "Invalid";
 
    // Return result
    return S;
}
 
// Driver Code
int main()
{
    string S = "MCMIV";
    cout << findLargest(S);
}

Java




// Java program for the above approach
import java.io.*;
import java.lang.*;
import java.util.*;
 
class GFG{
 
// Function to find decimal
// value of a roman character
static int charVal(char c)
{
    if (c == 'I')
        return 1;
    if (c == 'V')
        return 5;
    if (c == 'X')
        return 10;
    if (c == 'L')
        return 50;
    if (c == 'C')
        return 100;
    if (c == 'D')
        return 500;
    if (c == 'M')
        return 1000;
 
    return -1;
}
 
// Function to convert string representing
// roman number to equivalent decimal value
static int romanToDec(String S)
{
     
    // Stores the decimal value
    // of the string S
    int res = 0;
 
    // Stores the size of the S
    int n = S.length();
 
    // Update res
    res = charVal(S.charAt(n - 1));
 
    // Traverse the string
    for(int i = n - 2; i >= 0; i--)
    {
        if (charVal(S.charAt(i)) <
            charVal(S.charAt(i + 1)))
            res -= charVal(S.charAt(i));
        else
            res += charVal(S.charAt(i));
    }
 
    // Return res
    return res;
}
 
// Function to convert decimal
// to equivalent roman numeral
static String DecToRoman(int number)
{
     
    // Stores the string
    String res = "";
 
    // Stores all the digit values of a roman digit
    int num[] = { 14,   5,   9,   104050,
                  90, 100, 400, 500, 900, 1000 };
 
    String sym[] = { "I""IV", "V""IX",
                     "X""XL", "L", "XC",
                     "C""CD", "D""CM", "M" };
 
    int i = 12;
 
    // Iterate while number
    // is greater than 0
    while (number > 0)
    {
        int div = number / num[i];
        number = number % num[i];
         
        while (div-- > 0)
        {
            res += sym[i];
        }
        i--;
    }
 
    // Return res
    return res;
}
 
// Function to find largest roman
// value possible by rearranging
// the charcaters of the string
static String findLargest(String S)
{
    char arr[] = { 'I', 'V', 'X', 'L', 'C', 'D', 'M' };
 
    // Stores all roman characters
    Set<Character> st = new HashSet<>();
    for(char x : arr)
        st.add(x);
 
    Character s[] = new Character[S.length()];
    for(int i = 0; i < S.length(); i++)
        s[i] = S.charAt(i);
 
    // Traverse the string
    for(char x : s)
    {
         
        // If X is not found
        if (!st.contains(x))
            return "Invalid";
    }
 
    Arrays.sort(s, new Comparator<Character>()
    {
        @Override
        public int compare(Character x, Character y)
        {
            return charVal(y) - charVal(x);
        }
    });
 
    String ss = "";
    for(char ch : s)
        ss += ch;
 
    // Stores the decimal value
    // of the roman number
    int N = romanToDec(ss);
 
    // Find the roman value equivalent
    // to the decimal value of N
    String R = DecToRoman(N);
 
    if (!ss.equals(R))
        return "Invalid";
 
    // Return result
    return ss;
}
 
// Driver Code
public static void main(String[] args)
{
     
    // Input
    String S = "MCMIV";
    int N = S.length();
 
    System.out.println(findLargest(S));
}
}
 
// This code is contributed by Kingash
Output: 
MMCVI

 

Time Complexity: O(N*log(N))
Auxiliary Space: O(1)

 

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