一次交换中可能出现的字典序最大的字符串

给定长度为N 的字符串str ，任务是通过最多一次交换获得字典上最大的字符串。
注意：交换字符可能不相邻。

例子：

Input: str = “string” 
Output: tsring 
Explanation: 
Lexicographically largest string obtained by swapping string -> tsring.

Input: str = “zyxw” 
Output: zyxw 
Explanation: 
The given string is already lexicographically largest

方法：
为了解决上述问题，其主要思想是使用排序并计算给定字符串最大的辞书字符串可能。在排序从大到小的顺序给定的字符串后，找到指定的字符串的第一个无与伦比的字符和在排序字符串中的字符无法比拟的最后出现取代它。

Illustration: 
str = “geeks” 
Sorted string in descending order = “skgee”. 
The first unmatched character is in the first place. This character needs to be swapped with the character at this position in the sorted string which results in the lexicographically largest string. On replacing “g” with the “s”, the string obtained is “seekg” which is lexicographically largest after one swap.

下面是上述方法的实现：

C++

// C++ implementation to find the
// lexicographically largest string
// by atmost at most one swap
 
#include 
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());
 
    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

// 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();
}
 
// Funtion 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

# 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#

// 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

Javascript

 
    // 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));
     

输出：

ywstvr

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