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

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

// C++ program to print all occurrences of every character
// together.
import java.io.*;
import java.util.*;

class GFG
{
    // Since only lower case characters are there
    static int MAX_CHAR = 26;

    // Function 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];
        Arrays.fill(count,0);

        // 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));

            // Make this character's count value as 0.
            count[str.charAt(i)-'a'] = 0;
        }
    }

    // Driver Code
    public static void main(String[] args)
    {
        String str = "geeksforgeeks";

        printGrouped(str);
    }
}

// This code is contributed by shruti456rawal

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

Javascript

// javascript program to print all occurrences of every character
// together.

var MAX_CHAR = 26;

// Function to print the string
function printGrouped(str)
{
    var n = str.length;
    // Initialize counts of all characters as 0
    var count = Array(MAX_CHAR).fill(0);
    // Count occurrences of all characters in string
    for (var i=0; i < n; i++)
    {
        count[str.charAt(i).charCodeAt(0) - 'a'.charCodeAt(0)]++;
    }
    // Starts traversing the string
    for (var i=0; i < n; i++)
    {
        // Print the character till its count in
        // hash array
        while (count[str.charAt(i).charCodeAt(0) - 'a'.charCodeAt(0)]-- > 0)
        {
            console.log(str.charAt(i));
        }
        // Make this character's count value as 0.
        count[str.charAt(i).charCodeAt(0) - 'a'.charCodeAt(0)] = 0;
    }
}
// Driver Code

var str = "geeksforgeeks";
printGrouped(str);

// This code is contributed by Aarti_Rathi

Output

ggeeeekkssfor

Time complexity : O(n) 
Auxiliary Space : O(1)

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