# 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 smallest possible then print the generated string. **Examples:**

Input:str = “geek”, K = 2Output: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 = 5Output: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` |

**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