#### Combination

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```package com.thealgorithms.backtracking;

import java.util.Arrays;
import java.util.List;
import java.util.TreeSet;

/**
* Finds all permutations of given array
* @author Alan Piao (<a href="https://github.com/cpiao3">git-Alan Piao</a>)
*/
public final class Combination {
private Combination() {
}

private static int length;

/**
* Find all combinations of given array using backtracking
* @param arr the array.
* @param n length of combination
* @param <T> the type of elements in the array.
* @return a list of all combinations of length n. If n == 0, return null.
*/
public static <T> List<TreeSet<T>> combination(T[] arr, int n) {
if (n == 0) {
return null;
}
length = n;
T[] array = arr.clone();
Arrays.sort(array);
backtracking(array, 0, new TreeSet<T>(), result);
return result;
}

/**
* Backtrack all possible combinations of a given array
* @param arr the array.
* @param index the starting index.
* @param currSet set that tracks current combination
* @param result the list contains all combination.
* @param <T> the type of elements in the array.
*/
private static <T> void backtracking(T[] arr, int index, TreeSet<T> currSet, List<TreeSet<T>> result) {
if (index + length - currSet.size() > arr.length) return;
if (length - 1 == currSet.size()) {
for (int i = index; i < arr.length; i++) {