La string lexicográficamente más grande posible en un intercambio

Dada la string str de longitud N , la tarea es obtener la string lexicográficamente más grande mediante un intercambio como máximo. 

Nota: Es posible que los caracteres de intercambio no sean adyacentes.

Ejemplos:

Entrada: str = “string” 
Salida: tsring 
Explicación: 
La string lexicográficamente más grande obtenida intercambiando st ring -> ts ring.

Entrada: str = “zyxw” 
Salida: zyxw 
Explicación: 
La string dada ya es lexicográficamente más grande

Enfoque: 
Para resolver el problema mencionado anteriormente, la idea principal es utilizar Clasificación y calcular la string lexicográfica más grande posible para la string dada. Después de ordenar la string dada en orden descendente, busque el primer carácter no coincidente de la string dada y reemplácelo con la última aparición del carácter no coincidente en la string ordenada.

Ilustración: 
str = “geeks” 
String ordenada en orden descendente = “skgee”. 
El primer personaje inigualable está en primer lugar. Este carácter debe intercambiarse con el carácter en esta posición en la string ordenada, lo que da como resultado la string lexicográficamente más grande. Al reemplazar la “g” por la “s”, la string obtenida es “seekg”, que es lexicográficamente la más grande después de un intercambio.

A continuación se muestra la implementación del enfoque anterior:

// C++ implementation to find the
// lexicographically largest string
// by atmost at most one swap
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to return the
// lexicographically largest
// string possible by swapping
// at most one character
string findLargest(string s)
{
    int len = s.size();
 
    // Stores last occurrence
    // of every character
    int loccur[26];
 
    // Initialize with -1 for
    // every character
    memset(loccur, -1, sizeof(loccur));
 
    for (int i = len - 1; i >= 0; --i) {
 
        // Keep updating the last
        // occurrence of each character
        int chI = s[i] - 'a';
        // If a previously unvisited
        // character occurs
        if (loccur[chI] == -1) {
            loccur[chI] = i;
        }
    }
    // Stores the sorted string
    string sorted_s = s;
    sort(sorted_s.begin(), sorted_s.end(),
        greater<int>());
 
    for (int i = 0; i < len; ++i) {
        if (s[i] != sorted_s[i]) {
 
            // Character to replace
            int chI = sorted_s[i] - 'a';
 
            // Find the last occurrence
            // of this character
            int last_occ = loccur[chI];
 
            // Swap this with the last
            // occurrence
            swap(s[i], s[last_occ]);
            break;
        }
    }
 
    return s;
}
 
// Driver Program
int main()
{
    string s = "yrstvw";
    cout << findLargest(s);
    return 0;
}

// Java implementation to find the
// lexicographically largest string
// by atmost at most one swap
import java.util.*;
import java.lang.*;
 
class GFG{
     
// Function to return the
// lexicographically largest
// string possible by swapping
// at most one character
static String findLargest(StringBuilder s)
{
    int len = s.length();
     
    // Stores last occurrence
    // of every character
    int[] loccur = new int[26];
     
    // Initialize with -1 for
    // every character
    Arrays.fill(loccur, -1);
     
    for(int i = len - 1; i >= 0; --i)
    {
         
        // Keep updating the last
        // occurrence of each character
        int chI = s.charAt(i) - 'a';
         
        // If a previously unvisited
        // character occurs
        if (loccur[chI] == -1)
        {
            loccur[chI] = i;
        }
    }
     
    // Stores the sorted string
    char[] sorted_s = s.toString().toCharArray();
     
    Arrays.sort(sorted_s);
    reverse(sorted_s);
     
    for(int i = 0; i < len; ++i)
    {
        if (s.charAt(i) != sorted_s[i])
        {
             
            // Character to replace
            int chI = sorted_s[i] - 'a';
             
            // Find the last occurrence
            // of this character
            int last_occ = loccur[chI];
             
            // Swap this with the last
            // occurrence
            char tmp = s.charAt(i);
            s.setCharAt(i, s.charAt(last_occ));
            s.setCharAt(last_occ, tmp);
             
            break;
        }
    }
    return s.toString();
}
 
// Function to reverse array
static void reverse(char a[])
{
    int i, n = a.length;
     
    for(i = 0; i < n / 2; i++)
    {
        char t = a[i];
        a[i] = a[n - i - 1];
        a[n - i - 1] = t;
    }
}
 
// Driver Code
public static void main(String[] args)
{
    StringBuilder s = new StringBuilder("yrstvw");
     
    System.out.println(findLargest(s));
}
}
 
// This code is contributed by offbeat

# Python3 implementation to find the 
# lexicographically largest string 
# by atmost at most one swap
 
# Function to return the 
# lexicographically largest 
# string possible by swapping 
# at most one character
def findLargest(s):
    Len = len(s)
 
    # Stores last occurrence 
    # of every character
    # Initialize with -1 for 
    # every character
    loccur = [-1 for i in range(26)]
 
    for i in range(Len - 1, -1, -1):
 
        # Keep updating the last 
        # occurrence of each character
        chI = ord(s[i]) - ord('a')
 
        # If a previously unvisited 
        # character occurs
        if(loccur[chI] == -1):
            loccur[chI] = i
     
    # Stores the sorted string
    sorted_s = sorted(s, reverse = True)
    for i in range(Len):
        if(s[i] != sorted_s[i]):
 
            # Character to replace
            chI = (ord(sorted_s[i]) -
                   ord('a'))
 
            # Find the last occurrence 
            # of this character
            last_occ = loccur[chI]
            temp = list(s)
 
            # Swap this with the last 
            # occurrence
            temp[i], temp[last_occ] = (temp[last_occ],
                                       temp[i])
            s = "".join(temp)
            break
    return s
 
# Driver code
s = "yrstvw"
print(findLargest(s))
 
# This code is contributed by avanitrachhadiya2155

// C# implementation to find the
// lexicographically largest string
// by atmost at most one swap
using System;
using System.Collections.Generic;
 
class GFG{
     
// Function to return the
// lexicographically largest
// string possible by swapping
// at most one character
static string findLargest(char[] s)
{
    int len = s.Length;
      
    // Stores last occurrence
    // of every character
    int[] loccur = new int[26];
      
    // Initialize with -1 for
    // every character
    Array.Fill(loccur, -1);
      
    for(int i = len - 1; i >= 0; --i)
    {
         
        // Keep updating the last
        // occurrence of each character
        int chI = s[i] - 'a';
          
        // If a previously unvisited
        // character occurs
        if (loccur[chI] == -1)
        {
            loccur[chI] = i;
        }
    }
      
    // Stores the sorted string
    char[] sorted_s = (new string(s)).ToCharArray();
      
    Array.Sort(sorted_s);
    Array.Reverse(sorted_s);
     
    for(int i = 0; i < len; ++i)
    {
        if (s[i] != sorted_s[i])
        {
             
            // Character to replace
            int chI = sorted_s[i] - 'a';
  
            // Find the last occurrence
            // of this character
            int last_occ = loccur[chI];
  
            // Swap this with the last
            // occurrence
            char temp = s[i];
            s[i] = s[last_occ];
            s[last_occ] = temp;
            break;
        }
    }
    return (new string(s));
}
 
// Driver Code
static void Main()
{
    string str = "yrstvw";
    char[] s = str.ToCharArray();
     
    Console.WriteLine(findLargest(s));
}
}
 
// This code is contributed by divyesh072019

<script>
 
    // Javascript implementation to find the
    // lexicographically largest string
    // by atmost at most one swap
     
    // Function to return the
    // lexicographically largest
    // string possible by swapping
    // at most one character
    function findLargest(s)
    {
        let len = s.length;
 
        // Stores last occurrence
        // of every character
        let loccur = new Array(26);
 
        // Initialize with -1 for
        // every character
        loccur.fill(-1);
 
        for(let i = len - 1; i >= 0; --i)
        {
 
            // Keep updating the last
            // occurrence of each character
            let chI = s[i].charCodeAt() - 'a'.charCodeAt();
 
            // If a previously unvisited
            // character occurs
            if (loccur[chI] == -1)
            {
                loccur[chI] = i;
            }
        }
 
        // Stores the sorted string
        let sorted_s = s.join("").split('');
 
        sorted_s.sort();
        sorted_s.reverse();
 
        for(let i = 0; i < len; ++i)
        {
            if (s[i] != sorted_s[i])
            {
 
                // Character to replace
                let chI = sorted_s[i].charCodeAt() - 'a'.charCodeAt();
 
                // Find the last occurrence
                // of this character
                let last_occ = loccur[chI];
 
                // Swap this with the last
                // occurrence
                let temp = s[i];
                s[i] = s[last_occ];
                s[last_occ] = temp;
                break;
            }
        }
        return (s.join(""));
    }
     
    let str = "yrstvw";
    let s = str.split('');
      
    document.write(findLargest(s));
     
</script>

ywstvr