Lexicographically largest string formed from the characters in range L and R

Given a string S and a range L and R, the task is to print the lexicographically largest string that can be formed from the characters in range L and R.

Examples:

Input: str = "thgyfh", L = 2, R = 6  
Output: yhhgf

Input: str = "striver", L = 3, R = 5
Output: vri

Approach:

  • Iterate from min(L, R) to max(L, R) and increase the frequencies of characters in a freq[] array.
  • Iterate from 25 to 0 and print the number of times every character occurs to get the lexicographically largest string.

The common point of mistake which everyone does is they iterate from L to R instead of min(L, R) to max(L, R).

Below is the implementation of the above approach:

C++



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// C++ program to print the
// lexicographically largest string that
// can be formed from the characters
// in range L and R
  
#include <bits/stdc++.h>
using namespace std;
  
// Function to return the lexicographically largest string
string printLargestString(string s, int l, int r)
{
    // hash array
    int freq[26] = { 0 };
  
    // make 0-based indexing
    l--;
    r--;
  
    // iterate and count frequencies of character
    for (int i = min(l, r); i <= max(l, r); i++) {
        freq[s[i] - 'a']++;
    }
  
    // ans string
    string ans = "";
  
    // iterate in frequency array
    for (int i = 25; i >= 0; i--) {
  
        // add til all characters
        // are added
        while (freq[i]) {
            ans += char('a' + i);
            freq[i]--;
        }
    }
  
    return ans;
}
  
// Driver Code
int main()
{
    string s = "striver";
    int l = 3, r = 5;
    cout << printLargestString(s, l, r);
    return 0;
}

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Java














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// Java program to print the  


// lexicographically largest String that  


// can be formed from the characters  


// in range L and R   


  


class GFG { 


  


// Function to return the lexicographically largest String  


    static String printLargestString(String s, int l, int r) { 


        // hash array  


        int freq[] = new int[26]; 


  


        // make 0-based indexing  


        l--; 


        r--; 


  


        // iterate and count frequencies of character  


        for (int i = Math.min(l, r); i <= Math.max(l, r); i++) { 


            freq[s.charAt(i) - 'a']++; 


        } 


  


        // ans String  


        String ans = ""; 


  


        // iterate in frequency array  


        for (int i = 25; i >= 0; i--) { 


  


            // add til all characters  


            // are added  


            while (freq[i] > 0) { 


                ans += (char) ('a' + i); 


                freq[i]--; 


            } 


        } 


  


        return ans; 


    } 


  


// Driver Code  


    public static void main(String[] args) { 


  


        String s = "striver"; 


        int l = 3, r = 5; 


        System.out.println(printLargestString(s, l, r)); 


  


    } 


} 


/* This JAVA code is contributed by 29AjayKumar*/



















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Python 3














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# Python 3 program to print the 


# lexicographically largest string that 


# can be formed from the characters 


# in range L and R 


  


# Function to return the lexicographically  


# largest string 


def printLargestString(s, l, r): 


  


    # hash array 


    freq = [0] * 26


  


    # make 0-based indexing 


    l -= 1


    r -= 1


  


    # iterate and count frequencies of character 


    for i in range(min(l, r), max(l, r) + 1) : 


        freq[ord(s[i]) - ord('a')] += 1


  


    # ans string 


    ans = "" 


  


    # iterate in frequency array 


    for i in range(25, -1, -1): 


  


        # add til all characters are added 


        while (freq[i]): 


            ans += chr(ord('a') + i) 


            freq[i] -= 1


  


    return ans 


  


# Driver Code 


if __name__ == "__main__": 


      


    s = "striver"


    l = 3


    r = 5


    print(printLargestString(s, l, r)) 


  


# This code is contributed by ita_c 



















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C#














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// C# program to print the lexicographically 


// largest String that can be formed from the  


// characters in range L and R 


using System; 


  


class GFG 


{ 


  


// Function to return the lexicographically  


// largest String  


static String printLargestString(String s,  


                                 int l, int r)  


{ 


    // hash array  


    int []freq = new int[26]; 


  


    // make 0-based indexing  


    l--; 


    r--; 


  


    // iterate and count frequencies 


    // of character  


    for (int i = Math.Min(l, r);  


             i <= Math.Max(l, r); i++) 


    { 


        freq[s[i] - 'a']++; 


    } 


  


    // ans String  


    String ans = ""; 


  


    // iterate in frequency array  


    for (int i = 25; i >= 0; i--) 


    { 


  


        // add til all characters  


        // are added  


        while (freq[i] > 0)  


        { 


            ans += (char) ('a' + i); 


            freq[i]--; 


        } 


    } 


  


    return ans; 


} 


  


// Driver Code  


public static void Main()  


{ 


    String s = "striver"; 


    int l = 3, r = 5; 


    Console.Write(printLargestString(s, l, r)); 


} 


} 


  


// This code is contributed by 29AjayKumar 



















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Output:


vri





Time Complexity – O(N)

Each element gets added to the frequency table only once which takes O(1) and is appended to string which also takes O(1).


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