Group all occurrences of characters according to first appearance

Given a string of lowercase characters, the task is to print the string in a manner such that a character comes first in string displays first with all its occurrences in string.

Examples:

Input : str = "geeksforgeeks"
Output:  ggeeeekkssfor
Explanation: In the given string 'g' comes first 
and occurs 2 times so it is printed first
Then 'e' comes in this string and 4 times so 
it gets printed. Similarly remaining string is
printed.

Input :  str = "occurrence"
output : occcurreen 

Input  : str = "cdab"
Output : cdab

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

This problem is a string version of following problem for array of integers.

Group multiple occurrence of array elements ordered by first occurrences

Since given strings have only 26 possible characters, it is easier to implement for strings.

Implementation:
1- Count the occurrence of all the characters in given string using an array of size 26.
2- Then start traversing the string. Print every character its count times.

C++

// C++ program to print all occurrences of every character 
// together. 
# include<bits/stdc++.h> 
using namespace std; 
  
// Since only lower case characters are there 
const int MAX_CHAR = 26; 
  
// Function to print the string 
void printGrouped(string str) 
{ 
    int n = str.length(); 
  
    // Initialize counts of all characters as 0 
    int  count[MAX_CHAR] = {0}; 
  
    // Count occurrences of all characters in string 
    for (int i = 0 ; i < n ; i++) 
        count[str[i]-'a']++; 
  
    // Starts traversing the string 
    for (int i = 0; i < n ; i++) 
    { 
        // Print the character till its count in 
        // hash array 
        while (count[str[i]-'a']--) 
            cout << str[i]; 
  
        // Make this character's count value as 0. 
        count[str[i]-'a'] = 0; 
    } 
} 
  
// Driver code 
int main() 
{ 
    string str = "geeksforgeeks"; 
  
    printGrouped(str); 
  
    return 0; 
} 

Java

// Java program to print all occurrences of every character 
// together. 
  
class Test 
{ 
    // Since only lower case characters are there 
    static final int MAX_CHAR = 26; 
      
    // Method to print the string 
    static void printGrouped(String str) 
    { 
        int n = str.length(); 
       
        // Initialize counts of all characters as 0 
        int  count[] = new int[MAX_CHAR]; 
       
        // Count occurrences of all characters in string 
        for (int i = 0 ; i < n ; i++) 
            count[str.charAt(i)-'a']++; 
       
        // Starts traversing the string 
        for (int i = 0; i < n ; i++) 
        { 
            // Print the character till its count in 
            // hash array 
            while (count[str.charAt(i)-'a']!=0){ 
                System.out.print(str.charAt(i)); 
                count[str.charAt(i)-'a']--; 
            } 
       
            // Make this character's count value as 0. 
            count[str.charAt(i)-'a'] = 0; 
        } 
    } 
      
    // Driver method 
    public static void main(String args[]) 
    { 
        String str = new String("geeksforgeeks"); 
           
        printGrouped(str); 
    } 
} 

Python3

# Python3 program to print all occurrences 
# of every character together. 
  
# Since only lower case characters are there 
MAX_CHAR = 26
  
# Function to print the string 
def printGrouped(string): 
    n = len(string) 
  
    # Initialize counts of all characters as 0 
    count = [0] * MAX_CHAR 
  
    # Count occurrences of all characters in string 
    for i in range(n): 
        count[ord(string[i]) - ord("a")] += 1
  
    # Starts traversing the string 
    for i in range(n): 
  
        # Print the character till its count in 
        # hash array 
        while count[ord(string[i]) - ord("a")]: 
            print(string[i], end = "") 
            count[ord(string[i]) - ord("a")] -= 1
  
        # Make this character's count value as 0. 
        count[ord(string[i]) - ord("a")] = 0
  
# Driver code 
if __name__ == "__main__": 
    string = "geeksforgeeks"
    printGrouped(string) 
  
# This code is contributed by 
# sanjeev2552 

C#

// C# program to print all  
// occurrences of every  
// character together. 
using System; 
  
class GFG 
{ 
    // Since only lower case  
    // characters are there 
    static int MAX_CHAR = 26; 
      
    // Method to print  
    // the string 
    static void printGrouped(String str) 
    { 
        int n = str.Length; 
      
        // Initialize counts of 
        // all characters as 0 
        int []count = new int[MAX_CHAR]; 
      
        // Count occurrences of  
        // all characters in string 
        for (int i = 0 ; i < n ; i++) 
            count[str[i] - 'a']++; 
      
        // Starts traversing 
        // the string 
        for (int i = 0; i < n ; i++) 
        { 
            // Print the character  
            // till its count in 
            // hash array 
            while (count[str[i] - 'a'] != 0) 
            { 
                Console.Write(str[i]); 
                count[str[i] - 'a']--; 
            } 
      
            // Make this character's  
            // count value as 0. 
            count[str[i] - 'a'] = 0; 
        } 
    } 
      
    // Driver code 
    public static void Main() 
    { 
        string str = "geeksforgeeks"; 
        printGrouped(str); 
    } 
} 
  
// This code is contributed by Sam007 

Output:

ggeeeekkssfor

This article is contributed by Sahil Chhabra. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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My Personal Notes

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

C++

Java

Python3

C#

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