Given 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 = “neveropen”
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++
// 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
// 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 arraystatic 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 Codepublic 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 characterdef 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 codes = "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 Codestatic void Main(){ string str = "yrstvw"; char[] s = str.ToCharArray(); Console.WriteLine(findLargest(s)); }}// This code is contributed by divyesh072019 |
Javascript
<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> |
Output:
ywstvr
Time Complexity: O(n * log(n)), where n is the length of the given string.
Auxiliary Space: O(26) ? O(1), no extra space is required, so it is a constant.
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