# Sort elements by frequency (using Java Map)

Hello Everyone,

Given an integer array, sort the array according to the frequency of elements in decreasing order, if frequency of two elements are same then sort in increasing order

Examples:

Input: arr[] = {2, 3, 2, 4, 5, 12, 2, 3, 3, 3, 12}
Output: 3 3 3 3 2 2 2 12 12 4 5
Explanation :
No. Freq
2 : 3
3 : 4
4 : 1
5 : 1
12 : 2

Input: arr[] = {4, 4, 2, 2, 2, 2, 3, 3, 1, 1, 6, 7, 5}
Output: 2 2 2 2 1 1 3 3 4 4 5 6 7

Approach:

Java Map has been used in this set to solve the problem. The java.util.Map interface represents a mapping between a key and a value. The Map interface is not a subtype of the Collection interface. Therefore it behaves a bit different from the rest of the collection types.

In the below program:

• Get the element with its count in a Map
• By using the Comparator Interface, compare the frequency of an elements in a given list.
• Use this comparator to sort the list by implementing Collections.sort() method.
• Print the sorted list.

Program:

`import` `java.util.*;`

` `

`public` `class` `GFG {`

` `

` ` `// Driver Code`

` ` `public` `static` `void` `main(String[] args)`

` ` `{`

` `

` ` `// Declare and Initialize an array`

` ` `int` `[] array = { ` `4` `, ` `4` `, ` `2` `, ` `2` `, ` `2` `, ` `2` `, ` `3` `, ` `3` `, ` `1` `, ` `1` `, ` `6` `, ` `7` `, ` `5` `};`

` `

` ` `Map<Integer, Integer> map = ` `new` `HashMap<>();`

` ` `List<Integer> outputArray = ` `new` `ArrayList<>();`

` `

` ` `// Assign elements and their count in the list and map`

` ` `for` `(` `int` `current : array) {`

` ` `int` `count = map.getOrDefault(current, ` `0` `);`

` ` `map.put(current, count + ` `1` `);`

` ` `outputArray.add(current);`

` ` `}`

` `

` ` `// Compare the map by value`

` ` `SortComparator comp = ` `new` `SortComparator(map);`

` `

` ` `// Sort the map using Collections CLass`

` ` `Collections.sort(outputArray, comp);`

` `

` ` `// Final Output`

` ` `for` `(Integer i : outputArray) {`

` ` `System.out.print(i + ` `" "` `);`

` ` `}`

` ` `}`

`}`

` `

`// Implement Comparator Interface to sort the values`

`class` `SortComparator ` `implements` `Comparator<Integer> {`

` ` `private` `final` `Map<Integer, Integer> freqMap;`

` `

` ` `// Assign the specified map`

` ` `SortComparator(Map<Integer, Integer> tFreqMap)`

` ` `{`

` ` `this` `.freqMap = tFreqMap;`

` ` `}`

` `

` ` `// Compare the values`

` ` `@Override`

` ` `public` `int` `compare(Integer k1, Integer k2)`

` ` `{`

` `

` ` `// Compare value by frequency`

` ` `int` `freqCompare = freqMap.get(k2).compareTo(freqMap.get(k1));`

` `

` ` `// Compare value if frequency is equal`

` ` `int` `valueCompare = k1.compareTo(k2);`

` `

` ` `// If frequency is equal, then just compare by value, otherwise -`

` ` `// compare by the frequency.`

` ` `if` `(freqCompare == ` `0` `)`

` ` `return` `valueCompare;`

` ` `else`

` ` `return` `freqCompare;`

` ` `}`

`}`

Output:

2 2 2 2 1 1 3 3 4 4 5 6 7

Time Complexity : O(n Log n)