Lexicographically largest string possible in one swap

Given a string str of length N, the task is to obtain the lexicographically largest string by at most one swap.
Note: The swapping characters might not be adjacent.

Examples:

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.

Approach:
To solve the above-mentioned problem, the main idea is to use Sorting and compute the largest lexicographical string possible for the given string. After sorting the given string in descending order, find the first unmatched character from the given string and replace it with the last occurrence of the unmatched character in the sorted string.



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.

Below is the implementation of the above approach:

C++

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// 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;
}

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Output:

ywstvr

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