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refactor: change List<Integer> to LinkedList<Integer> and remove 'n < 0' #5439

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refactor: change List<Integer> to LinkedList<Integer> and remove 'n < 0' #5439

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7 changes: 4 additions & 3 deletions src/main/java/com/thealgorithms/backtracking/ArrayCombination.java
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
package com.thealgorithms.backtracking;

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;

/**
Expand All @@ -18,16 +19,16 @@ private ArrayCombination() {
* @return A list of all combinations of length k.
*/
public static List<List<Integer>> combination(int n, int k) {
if (n < 0 || k < 0 || k > n) {
if (k < 0 || k > n) {
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The n < 0 check is important because it ensures that the input values are valid within the context of combinations, prevents undefined behavior.

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It looks like if you remove this check everything will work correctly. And you're right, we can remove it. But this check provides readable constraints and with this check the code becomes more readable.

throw new IllegalArgumentException("Wrong input.");
}

List<List<Integer>> combinations = new ArrayList<>();
combine(combinations, new ArrayList<>(), 0, n, k);
combine(combinations, new LinkedList<>(), 0, n, k);
return combinations;
}

private static void combine(List<List<Integer>> combinations, List<Integer> current, int start, int n, int k) {
private static void combine(List<List<Integer>> combinations, LinkedList<Integer> current, int start, int n, int k) {
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Using LinkedList instead of List in method parameters is considered bad practice as it provides tight coupling, and issues with extensibility.

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List doesn't seem to have a removeLast method

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It has. In code below, you can see that it already used. And code compile and pass tests.

if (current.size() == k) { // Base case: combination found
combinations.add(new ArrayList<>(current)); // Copy to avoid modification
return;
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