diff --git a/DIRECTORY.md b/DIRECTORY.md index 228735aa8ea9..9fb1112924f3 100644 --- a/DIRECTORY.md +++ b/DIRECTORY.md @@ -267,6 +267,7 @@ * [SumOfSubset](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/dynamicprogramming/SumOfSubset.java) * [Tribonacci](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/dynamicprogramming/Tribonacci.java) * [UniquePaths](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/dynamicprogramming/UniquePaths.java) + * [UniqueSubsequencesCount](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/dynamicprogramming/UniqueSubsequencesCount.java) * [WildcardMatching](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/dynamicprogramming/WildcardMatching.java) * [WineProblem](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/dynamicprogramming/WineProblem.java) * geometry @@ -566,6 +567,7 @@ * [NextSmallerElement](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/NextSmallerElement.java) * [PostfixToInfix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PostfixToInfix.java) * [PrefixToInfix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PrefixToInfix.java) + * [SortStack](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/SortStack.java) * [StackPostfixNotation](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/StackPostfixNotation.java) * strings * [AhoCorasick](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/strings/AhoCorasick.java) @@ -794,6 +796,7 @@ * [SumOfSubsetTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/dynamicprogramming/SumOfSubsetTest.java) * [TribonacciTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/dynamicprogramming/TribonacciTest.java) * [UniquePathsTests](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/dynamicprogramming/UniquePathsTests.java) + * [UniqueSubsequencesCountTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/dynamicprogramming/UniqueSubsequencesCountTest.java) * [WildcardMatchingTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/dynamicprogramming/WildcardMatchingTest.java) * geometry * [GrahamScanTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/geometry/GrahamScanTest.java) @@ -1027,6 +1030,7 @@ * [NextSmallerElementTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/NextSmallerElementTest.java) * [PostfixToInfixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PostfixToInfixTest.java) * [PrefixToInfixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PrefixToInfixTest.java) + * [SortStackTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/SortStackTest.java) * [StackPostfixNotationTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/StackPostfixNotationTest.java) * strings * [AhoCorasickTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/strings/AhoCorasickTest.java) diff --git a/src/main/java/com/thealgorithms/stacks/SortStack.java b/src/main/java/com/thealgorithms/stacks/SortStack.java new file mode 100644 index 000000000000..d07d1a5f1dc3 --- /dev/null +++ b/src/main/java/com/thealgorithms/stacks/SortStack.java @@ -0,0 +1,60 @@ +package com.thealgorithms.stacks; + +import java.util.Stack; + +/** + * A utility class that provides a method to sort a stack using recursion. + * The elements are sorted in ascending order, with the largest element at the top. + * This algorithm is implemented using only recursion and the original stack, + * without utilizing any additional data structures apart from the stack itself. + */ +public final class SortStack { + private SortStack() { + } + + /** + * Sorts the given stack in ascending order using recursion. + * The sorting is performed such that the largest element ends up on top of the stack. + * This method modifies the original stack and does not return a new stack. + * + * The algorithm works as follows: + * 1. Remove the top element. + * 2. Recursively sort the remaining stack. + * 3. Insert the removed element back into the sorted stack at the correct position. + * + * @param stack The stack to be sorted, containing Integer elements. + * @throws IllegalArgumentException if the stack contains `null` elements. + */ + public static void sortStack(Stack stack) { + if (stack.isEmpty()) { + return; + } + + int top = stack.pop(); + sortStack(stack); + insertInSortedOrder(stack, top); + } + + /** + * Helper method to insert an element into the correct position in a sorted stack. + * This method is called recursively to place the given element into the stack + * such that the stack remains sorted in ascending order. + * + * The element is inserted in such a way that all elements below it are smaller + * (if the stack is non-empty), and elements above it are larger, maintaining + * the ascending order. + * + * @param stack The stack in which the element needs to be inserted. + * @param element The element to be inserted into the stack in sorted order. + */ + private static void insertInSortedOrder(Stack stack, int element) { + if (stack.isEmpty() || element > stack.peek()) { + stack.push(element); + return; + } + + int top = stack.pop(); + insertInSortedOrder(stack, element); + stack.push(top); + } +} diff --git a/src/test/java/com/thealgorithms/stacks/SortStackTest.java b/src/test/java/com/thealgorithms/stacks/SortStackTest.java new file mode 100644 index 000000000000..b9f2f1b6f106 --- /dev/null +++ b/src/test/java/com/thealgorithms/stacks/SortStackTest.java @@ -0,0 +1,77 @@ +package com.thealgorithms.stacks; + +import static org.junit.jupiter.api.Assertions.assertEquals; +import static org.junit.jupiter.api.Assertions.assertTrue; + +import java.util.Stack; +import org.junit.jupiter.api.BeforeEach; +import org.junit.jupiter.api.Test; + +public class SortStackTest { + + private Stack stack; + + @BeforeEach + public void setUp() { + stack = new Stack<>(); + } + + @Test + public void testSortEmptyStack() { + SortStack.sortStack(stack); + assertTrue(stack.isEmpty()); // An empty stack should remain empty + } + + @Test + public void testSortSingleElementStack() { + stack.push(10); + SortStack.sortStack(stack); + assertEquals(1, stack.size()); + assertEquals(10, (int) stack.peek()); // Single element should remain unchanged + } + + @Test + public void testSortAlreadySortedStack() { + stack.push(1); + stack.push(2); + stack.push(3); + stack.push(4); + SortStack.sortStack(stack); + + assertEquals(4, stack.size()); + assertEquals(4, (int) stack.pop()); + assertEquals(3, (int) stack.pop()); + assertEquals(2, (int) stack.pop()); + assertEquals(1, (int) stack.pop()); + } + + @Test + public void testSortUnsortedStack() { + stack.push(3); + stack.push(1); + stack.push(4); + stack.push(2); + SortStack.sortStack(stack); + + assertEquals(4, stack.size()); + assertEquals(4, (int) stack.pop()); + assertEquals(3, (int) stack.pop()); + assertEquals(2, (int) stack.pop()); + assertEquals(1, (int) stack.pop()); + } + + @Test + public void testSortWithDuplicateElements() { + stack.push(3); + stack.push(1); + stack.push(3); + stack.push(2); + SortStack.sortStack(stack); + + assertEquals(4, stack.size()); + assertEquals(3, (int) stack.pop()); + assertEquals(3, (int) stack.pop()); + assertEquals(2, (int) stack.pop()); + assertEquals(1, (int) stack.pop()); + } +}