Peak Elements

core-java-modules/core-java-collections-array-list-2/src/main/java/com/baeldung/peakelements/MultiplePeakFinder.java

@@ -0,0 +1,40 @@
+package com.baeldung.peakelements;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class MultiplePeakFinder {
+
+    public static List<Integer> findPeaks(int[] arr) {
+
+        List<Integer> peaks = new ArrayList<>();
+
+        if (arr == null || arr.length == 0) {
+            return peaks;
+        }
+        findPeakElement(arr, 0, arr.length - 1, peaks);
+        return peaks;
+    }
+
+    private static void findPeakElement(int[] arr, int low, int high, List<Integer> peaks) {
+
+        if (low > high) {
+            return;
+        }
+
+        int mid = low + (high - low) / 2;
+
+        if ((mid == 0 || arr[mid] >= arr[mid - 1]) && (mid == arr.length - 1 || arr[mid] >= arr[mid + 1])) {
+            peaks.add(arr[mid]);
+        }
+
+        if (mid > 0) {
+            findPeakElement(arr, low, mid - 1, peaks);
+        }
+
+        if (mid < arr.length - 1) {
+            findPeakElement(arr, mid + 1, high, peaks);
+        }
+
+    }
+}

core-java-modules/core-java-collections-array-list-2/src/main/java/com/baeldung/peakelements/PeakElementFinder.java

@@ -0,0 +1,38 @@
+package com.baeldung.peakelements;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class PeakElementFinder {
+    public List<Integer> findPeakElement(int[] arr) {
+        int n = arr.length;
+        List<Integer> peaks = new ArrayList<>();
+
+        if (n == 0) {
+            return peaks;
+        }
+
+        for (int i = 0; i < n; i++) {
+            if (isPeak(arr, i, n)) {
+                peaks.add(i);
+            }
+            while (i < n - 1 && arr[i] == arr[i + 1]) {
+                i++;
+            }
+        }
+
+        return peaks;
+    }
+
+    private boolean isPeak(int[] arr, int index, int n) {
+
+        if (index == 0) {
+            return n > 1 ? arr[index] >= arr[index + 1] : true;
+        } else if (index == n - 1) {
+            return arr[index] >= arr[index - 1];
+        }
+
+        return arr[index] >= arr[index - 1] && arr[index] >= arr[index + 1];
+    }
+}
+

core-java-modules/core-java-collections-array-list-2/src/main/java/com/baeldung/peakelements/SinglePeakFinder.java

@@ -0,0 +1,25 @@
+package com.baeldung.peakelements;
+
+public class SinglePeakFinder {
+
+    public static int findSinglePeak(int[] arr) {
+        int n = arr.length;
+
+        for (int i = 1; i < n - 1; i++) {
+            if (arr[i] > arr[i - 1] && arr[i] > arr[i + 1]) {
+                return arr[i];
+            }
+        }
+
+        if (arr[0] > arr[1]) {
+            return arr[0];
+        }
+
+        if (arr[n - 1] > arr[n - 2]) {
+            return arr[n - 1];
+        }
+
+        return -1;
+    }
+}
+

core-java-modules/core-java-collections-array-list-2/src/test/java/com/baeldung/peakelements/MultiplePeakFinderUnitTest.java

@@ -0,0 +1,24 @@
+package com.baeldung.peakelements;
+
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class MultiplePeakFinderUnitTest {
+
+    @Test
+    void findPeaks_givenArrayOfIntegers_whenValidInput_thenReturnsCorrectPeaks() {
+        MultiplePeakFinder finder = new MultiplePeakFinder();
+        int[] array = {1, 13, 7, 0, 4, 1, 4, 45, 50};
+        List<Integer> peaks = finder.findPeaks(array);
+
+        assertEquals(3, peaks.size());
+        assertTrue(peaks.contains(4));
+        assertTrue(peaks.contains(13));
+        assertTrue(peaks.contains(50));
+    }
+}
+

core-java-modules/core-java-collections-array-list-2/src/test/java/com/baeldung/peakelements/PeakElementFinderUnitTest.java

@@ -0,0 +1,42 @@
+package com.baeldung.peakelements;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class PeakElementFinderUnitTest {
+
+    @Test
+    void findPeakElement_givenArrayOfIntegers_whenValidInput_thenReturnsCorrectPeakIndex() {
+        PeakElementFinder finder = new PeakElementFinder();
+        int[] array = {5, 8, 7, 6, 12};
+        List<Integer> peaks = finder.findPeakElement(array);
+        assertEquals(2, peaks.size() );
+    }
+
+    @Test
+    void findPeakElement_givenArrayOfIntegers_whenNoPeaks_thenReturnsMinusOne() {
+        PeakElementFinder finder = new PeakElementFinder();
+        int[] array = {5, 8, 7, 6, 12};
+        List<Integer> peaks = finder.findPeakElement(array);
+        assertEquals(2, peaks.size());
+    }
+
+    @Test
+    void findPeakElement_givenArrayOfIntegers_whenPeaksAtExtremes_thenReturnsCorrectPeakIndex() {
+        PeakElementFinder finder = new PeakElementFinder();
+        int[] array = {5, 2, 1, 3, 4};
+        List<Integer> peaks = finder.findPeakElement(array);
+        assertEquals(2, peaks.size());
+    }
+
+    @Test
+    void findPeakElement_givenArrayOfIntegers_whenPlateaus_thenReturnsCorrectPeakIndex() {
+        PeakElementFinder finder = new PeakElementFinder();
+        int[] array = {1, 2, 2, 2, 3, 4, 5};
+        List<Integer> peaks = finder.findPeakElement(array);
+        assertEquals(2, peaks.size());
+    }
+}

core-java-modules/core-java-collections-array-list-2/src/test/java/com/baeldung/peakelements/SinglePeakFinderUnitTest.java

@@ -0,0 +1,13 @@
+package com.baeldung.peakelements;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import org.junit.jupiter.api.Test;
+
+public class SinglePeakFinderUnitTest {
+
+    @Test
+    void findSinglePeak_givenArrayOfIntegers_whenValidInput_thenReturnsCorrectPeak() {
+        int[] arr = {0, 10, 2, 4, 5, 1};
+        assertEquals(10, SinglePeakFinder.findSinglePeak(arr));
+    }
+}