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Merge branch 'master' into array_combination_improve
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‎DIRECTORY.md

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* [WiggleSort](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/sorts/WiggleSort.java)
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* stacks
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* [BalancedBrackets](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/BalancedBrackets.java)
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* [CelebrityFinder](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/CelebrityFinder.java)
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* [DecimalToAnyUsingStack](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/DecimalToAnyUsingStack.java)
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* [DuplicateBrackets](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/DuplicateBrackets.java)
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* [InfixToPostfix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/InfixToPostfix.java)
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* [InfixToPrefix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/InfixToPrefix.java)
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* [LargestRectangle](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/LargestRectangle.java)
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* [MaximumMinimumWindow](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/MaximumMinimumWindow.java)
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* [MinStackUsingSingleStack](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/MinStackUsingSingleStack.java)
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* [MinStackUsingTwoStacks](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/MinStackUsingTwoStacks.java)
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* [NextGreaterElement](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/NextGreaterElement.java)
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* [NextSmallerElement](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/NextSmallerElement.java)
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* [PostfixEvaluator](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PostfixEvaluator.java)
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* [WiggleSortTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/sorts/WiggleSortTest.java)
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* stacks
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* [BalancedBracketsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/BalancedBracketsTest.java)
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* [CelebrityFinderTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/CelebrityFinderTest.java)
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* [DecimalToAnyUsingStackTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/DecimalToAnyUsingStackTest.java)
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* [DuplicateBracketsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/DuplicateBracketsTest.java)
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* [InfixToPostfixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/InfixToPostfixTest.java)
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* [InfixToPrefixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/InfixToPrefixTest.java)
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* [LargestRectangleTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/LargestRectangleTest.java)
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* [MinStackUsingSingleStackTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/MinStackUsingSingleStackTest.java)
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* [MinStackUsingTwoStacksTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/MinStackUsingTwoStacksTest.java)
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* [NextGreaterElementTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/NextGreaterElementTest.java)
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* [NextSmallerElementTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/NextSmallerElementTest.java)
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* [PostfixEvaluatorTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PostfixEvaluatorTest.java)

‎src/main/java/com/thealgorithms/stacks/CelebrityFinder.java

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package com.thealgorithms.stacks;
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import java.util.Stack;
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/**
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* Solves the celebrity problem using a stack-based algorithm.
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*
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* <p>Celebrity is someone known by everyone but doesn't know anyone else.
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* <p>Applications: Graph theory and social network analysis.
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*
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* @author Hardvan
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*/
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public final class CelebrityFinder {
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private CelebrityFinder() {
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}
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/**
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* Finds the celebrity in the given party matrix using a stack-based algorithm.
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*
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* @param party A 2D matrix where party[i][j] is 1 if i knows j, otherwise 0.
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* @return The index of the celebrity, or -1 if there is no celebrity.
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*/
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public static int findCelebrity(int[][] party) {
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// Push all people onto the stack
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Stack<Integer> stack = new Stack<>();
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for (int i = 0; i < party.length; i++) {
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stack.push(i);
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}
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// Find the potential celebrity by comparing pairs
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while (stack.size() > 1) {
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int person1 = stack.pop();
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int person2 = stack.pop();
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if (party[person1][person2] == 1) {
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stack.push(person2); // person1 knows person2, so person2 might be the celebrity
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} else {
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stack.push(person1); // person1 doesn't know person2, so person1 might be the celebrity
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}
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}
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// Verify the candidate
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int candidate = stack.pop();
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for (int i = 0; i < party.length; i++) {
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if (i != candidate && (party[candidate][i] == 1 || party[i][candidate] == 0)) {
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return -1;
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}
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}
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return candidate;
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}
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}

‎src/main/java/com/thealgorithms/stacks/MinStackUsingTwoStacks.java

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package com.thealgorithms.stacks;
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import java.util.Stack;
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/**
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* Min-Stack implementation that supports push, pop, and retrieving the minimum element in constant time.
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*
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* @author Hardvan
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*/
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public final class MinStackUsingTwoStacks {
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MinStackUsingTwoStacks() {
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}
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private final Stack<Integer> stack = new Stack<>();
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private final Stack<Integer> minStack = new Stack<>();
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/**
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* Pushes a new element onto the {@code stack}.
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* If the value is less than or equal to the current minimum, it is also pushed onto the {@code minStack}.
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*
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* @param value The value to be pushed.
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*/
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public void push(int value) {
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stack.push(value);
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if (minStack.isEmpty() || value <= minStack.peek()) {
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minStack.push(value);
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}
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}
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/**
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* Removes the top element from the stack.
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* If the element is the minimum element, it is also removed from the {@code minStack}.
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*/
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public void pop() {
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if (stack.pop().equals(minStack.peek())) {
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minStack.pop();
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}
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}
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/**
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* Retrieves the top element of the stack.
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*
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* @return The top element.
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*/
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public int top() {
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return stack.peek();
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}
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/**
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* Retrieves the minimum element in the stack.
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*
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* @return The minimum element.
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*/
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public int getMin() {
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return minStack.peek();
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}
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}

‎src/test/java/com/thealgorithms/stacks/CelebrityFinderTest.java

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package com.thealgorithms.stacks;
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import static org.junit.jupiter.api.Assertions.assertEquals;
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import java.util.stream.Stream;
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import org.junit.jupiter.params.ParameterizedTest;
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import org.junit.jupiter.params.provider.Arguments;
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import org.junit.jupiter.params.provider.MethodSource;
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public class CelebrityFinderTest {
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@ParameterizedTest
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@MethodSource("providePartyMatrices")
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public void testCelebrityFinder(int[][] party, int expected) {
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assertEquals(expected, CelebrityFinder.findCelebrity(party));
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}
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private static Stream<Arguments> providePartyMatrices() {
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return Stream.of(
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// Test case 1: Celebrity exists
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Arguments.of(new int[][] {{0, 1, 1}, {0, 0, 1}, {0, 0, 0}}, 2),
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// Test case 2: No celebrity
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Arguments.of(new int[][] {{0, 1, 0}, {1, 0, 1}, {1, 1, 0}}, -1),
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// Test case 3: Everyone knows each other, no celebrity
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Arguments.of(new int[][] {{0, 1, 1}, {1, 0, 1}, {1, 1, 0}}, -1),
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// Test case 4: Single person, they are trivially a celebrity
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Arguments.of(new int[][] {{0}}, 0),
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// Test case 5: All know the last person, and they know no one
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Arguments.of(new int[][] {{0, 1, 1, 1}, {0, 0, 1, 1}, {0, 0, 0, 1}, {0, 0, 0, 0}}, 3),
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// Test case 6: Larger party with no celebrity
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Arguments.of(new int[][] {{0, 1, 1, 0}, {1, 0, 0, 1}, {0, 1, 0, 1}, {1, 1, 1, 0}}, -1),
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// Test case 7: Celebrity at the start of the matrix
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Arguments.of(new int[][] {{0, 0, 0}, {1, 0, 1}, {1, 1, 0}}, 0));
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}
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}

‎src/test/java/com/thealgorithms/stacks/MinStackUsingTwoStacksTest.java

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package com.thealgorithms.stacks;
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import static org.junit.jupiter.api.Assertions.assertEquals;
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import org.junit.jupiter.api.Test;
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public class MinStackUsingTwoStacksTest {
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@Test
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public void testMinStackOperations() {
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MinStackUsingTwoStacks minStack = new MinStackUsingTwoStacks();
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minStack.push(3);
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minStack.push(5);
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assertEquals(3, minStack.getMin());
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minStack.push(2);
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minStack.push(1);
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assertEquals(1, minStack.getMin());
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minStack.pop();
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assertEquals(2, minStack.getMin());
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}
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@Test
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public void testMinStackOperations2() {
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MinStackUsingTwoStacks minStack = new MinStackUsingTwoStacks();
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minStack.push(3);
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minStack.push(5);
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assertEquals(3, minStack.getMin());
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minStack.push(2);
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minStack.push(1);
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assertEquals(1, minStack.getMin());
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minStack.pop();
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assertEquals(2, minStack.getMin());
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}
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}

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