Lexicographically smallest string formed by appending a character from first K characters of a string | Set 2
Given a string str consisting of lowercase alphabets and an integer K, you can perform the following operations on str
- Initialize an empty string X = “”.
- Take any character from the first K characters of str and append it to X.
- Remove the chosen character from str.
- Repeat the above steps while there are characters left in str.
The task is to generate X such that it is lexicographically the smallest possible then print the generated string. Examples:
Input: str = “geek”, K = 2
Output: eegk Operation 1: str = “gek”, X = “e” Operation 2: str = “gk”, X = “ee” Operation 3: str = “k”, X = “eeg” Operation 4: str = “”, X = “eegk”
Input: str = “w3wiki”, K = 5
Output: eefggeekkorss
Approach: In order to get the lexicographically smallest string, we need to take the minimum character from the first K characters every time we choose a character from str. To do that, we can put the first K characters in a priority_queue (min-heap) and then choose the smallest character and append it to X. Then, push the next character in str to the priority queue and repeat the process until there are characters left to process. Below is the implementation of the above approach:
C++
// C++ implementation of the approach #include <bits/stdc++.h> using namespace std; // Function to return the lexicographically // smallest required string string getSmallestStr(string S, int K) { // Initially empty string string X = "" ; // min heap of characters priority_queue< char , vector< char >, greater< char > > pq; // Length of the string int i, n = S.length(); // K cannot be greater than // the size of the string K = min(K, n); // First push the first K characters // into the priority_queue for (i = 0; i < K; i++) pq.push(S[i]); // While there are characters to append while (!pq.empty()) { // Append the top of priority_queue to X X += pq.top(); // Remove the top element pq.pop(); // Push only if i is less than // the size of string if (i < S.length()) pq.push(S[i]); i++; } // Return the generated string return X; } // Driver code int main() { string S = "w3wiki" ; int K = 5; cout << getSmallestStr(S, K); return 0; } |
Java
// Java implementation of the approach import java.util.PriorityQueue; class GFG { // Function to return the lexicographically // smallest required string static String getSmallestStr(String S, int K) { // Initially empty string String X = "" ; // min heap of characters PriorityQueue<Character> pq = new PriorityQueue<>(); // Length of the string int i, n = S.length(); // K cannot be greater than // the size of the string K = Math.min(K, n); // First push the first K characters // into the priority_queue for (i = 0 ; i < K; i++) pq.add(S.charAt(i)); // While there are characters to append while (!pq.isEmpty()) { // Append the top of priority_queue to X X += pq.peek(); // Remove the top element pq.remove(); // Push only if i is less than // the size of string if (i < S.length()) pq.add(S.charAt(i)); i++; } // Return the generated string return X; } // Driver Code public static void main(String[] args) { String S = "w3wiki" ; int K = 5 ; System.out.println(getSmallestStr(S, K)); } } // This code is contributed by // sanjeev2552 |
C#
using System; using System.Collections.Generic; namespace GetSmallestString { class Program { static string GetSmallestStr( string S, int K) { // Initially empty string string X = "" ; // min heap of characters var pq = new SortedSet< char >(); // Length of the string int i, n = S.Length; // K cannot be greater than // the size of the string K = Math.Min(K, n); // First push the first K characters // into the priority_queue for (i = 0; i < K; i++) pq.Add(S[i]); // While there are characters to append while (pq.Count > 0) { // Append the top of priority_queue to X X += pq.Min; // Remove the top element pq.Remove(pq.Min); // Push only if i is less than // the size of string if (i < S.Length) pq.Add(S[i]); i++; } // Return the generated string return X; } static void Main( string [] args) { string S = "w3wiki" ; int K = 5; Console.WriteLine(GetSmallestStr(S, K)); } } } // this code is contributed by writer |
Javascript
// JavaScript implementation of the approach class GFG { // Function to return the lexicographically // smallest required string static getSmallestStr(S, K) { // Initially empty string let X = "" ; // min heap of characters let pq = []; // Length of the string let i, n = S.length; // K cannot be greater than // the size of the string K = Math.min(K, n); // First push the first K characters // into the priority_queue for (i = 0; i < K; i++) { pq.push(S.charAt(i)); } // Sort the priority queue in ascending order pq.sort(); // While there are characters to append while (pq.length > 0) { // Append the top of priority_queue to X X += pq[0]; // Remove the top element pq.shift(); // Push only if i is less than // the size of string if (i < S.length) { pq.push(S.charAt(i)); } i++; // Sort the priority queue in ascending order pq.sort(); } // Return the generated string return X; } // Driver Code static main() { let S = "w3wiki" ; let K = 5; console.log(GFG.getSmallestStr(S, K)); } } GFG.main(); |
Python3
import heapq def get_smallest_str(s: str , k: int ) - > str : # Initialize an empty string x = "" # Create a heap of characters pq = [] # Get the length of the string n = len (s) # k cannot be greater than the size of the string k = min (k, n) # First push the first k characters into the priority_queue for i in range (k): heapq.heappush(pq, s[i]) # While there are characters to append i = k while pq: # Append the top of priority_queue to x x + = heapq.heappop(pq) # Push only if i is less than the size of string if i < n: heapq.heappush(pq, s[i]) i + = 1 # Return the generated string return x # Driver code s = "w3wiki" k = 5 print (get_smallest_str(s, k)) |
eefggeekkorss
Time Complexity: O(nlogn) where n is the length of the string.
Auxiliary Space: O(K), as extra space of size K is used to build priorityQueue