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

1. Initialize an empty string X = “”.
2. Take any character from the first K characters of str and append it to X.
3. Remove the chosen character from str.
4. 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 = “geeksforgeeks”, 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 = "geeksforgeeks";
    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 = "geeksforgeeks";
        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 = "geeksforgeeks";
            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 = "geeksforgeeks";
    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 = "geeksforgeeks"
k = 5
print(get_smallest_str(s, k))

Output

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