Add Merge Intervals algorithm

Author: Hardvan

src/main/java/com/thealgorithms/greedyalgorithms/MergeIntervals.java

@@ -0,0 +1,64 @@
+package com.thealgorithms.greedyalgorithms;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+/**
+ * Problem Statement:
+ * Given an array of intervals where intervals[i] = [starti, endi].
+ * 
+ * Merge all overlapping intervals and return an array of the non-overlapping
+ * intervals
+ * that cover all the intervals in the input.
+ */
+public final class MergeIntervals {
+
+    /**
+     * Private constructor to prevent instantiation of this utility class.
+     */
+    private MergeIntervals() {
+    }
+
+    /**
+     * Merges overlapping intervals from the given array of intervals.
+     * 
+     * The method sorts the intervals by their start time, then iterates through the
+     * sorted intervals
+     * and merges overlapping intervals. If an interval overlaps with the last
+     * merged interval,
+     * it updates the end time of the last merged interval. Otherwise, it adds the
+     * interval as a new entry.
+     * 
+     * @param intervals A 2D array representing intervals where each element is an
+     *                  interval [starti, endi].
+     * @return A 2D array of merged intervals where no intervals overlap.
+     * 
+     *         Example:
+     *         Input: {{1, 3}, {2, 6}, {8, 10}, {15, 18}}
+     *         Output: {{1, 6}, {8, 10}, {15, 18}}
+     */
+    public static int[][] merge(int[][] intervals) {
+        // Sort the intervals by their start time (ascending order)
+        Arrays.sort(intervals, (a, b) -> Integer.compare(a[0], b[0]));
+
+        // List to store merged intervals
+        List<int[]> merged = new ArrayList<>();
+
+        for (int[] interval : intervals) { // Each interval
+            // If the merged list is empty or the current interval does not overlap with
+            // the last merged interval, add it to the merged list.
+            if (merged.isEmpty() || interval[0] > merged.get(merged.size() - 1)[1]) {
+                merged.add(interval);
+            } else {
+                // If there is an overlap, merge the intervals by updating the end time
+                // of the last merged interval to the maximum end time between the two
+                // intervals.
+                merged.get(merged.size() - 1)[1] = Math.max(merged.get(merged.size() - 1)[1], interval[1]);
+            }
+        }
+
+        // Convert the list of merged intervals back to a 2D array and return it
+        return merged.toArray(new int[merged.size()][]);
+    }
+}

src/test/java/com/thealgorithms/greedyalgorithms/MergeIntervalsTest.java

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+package com.thealgorithms.greedyalgorithms;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class MergeIntervalsTest {
+
+    @Test
+    public void testMergeIntervalsWithOverlappingIntervals() {
+        // Test case where some intervals overlap and should be merged
+        int[][] intervals = { { 1, 3 }, { 2, 6 }, { 8, 10 }, { 15, 18 } };
+        int[][] expected = { { 1, 6 }, { 8, 10 }, { 15, 18 } };
+        int[][] result = MergeIntervals.merge(intervals);
+        assertArrayEquals(expected, result);
+    }
+
+    @Test
+    public void testMergeIntervalsWithNoOverlap() {
+        // Test case where intervals do not overlap
+        int[][] intervals = { { 1, 2 }, { 3, 4 }, { 5, 6 } };
+        int[][] expected = { { 1, 2 }, { 3, 4 }, { 5, 6 } };
+        int[][] result = MergeIntervals.merge(intervals);
+        assertArrayEquals(expected, result);
+    }
+
+    @Test
+    public void testMergeIntervalsWithCompleteOverlap() {
+        // Test case where intervals completely overlap
+        int[][] intervals = { { 1, 5 }, { 2, 4 }, { 3, 6 } };
+        int[][] expected = { { 1, 6 } };
+        int[][] result = MergeIntervals.merge(intervals);
+        assertArrayEquals(expected, result);
+    }
+
+    @Test
+    public void testMergeIntervalsWithSingleInterval() {
+        // Test case where only one interval is given
+        int[][] intervals = { { 1, 2 } };
+        int[][] expected = { { 1, 2 } };
+        int[][] result = MergeIntervals.merge(intervals);
+        assertArrayEquals(expected, result);
+    }
+
+    @Test
+    public void testMergeIntervalsWithEmptyArray() {
+        // Test case where the input array is empty
+        int[][] intervals = {};
+        int[][] expected = {};
+        int[][] result = MergeIntervals.merge(intervals);
+        assertArrayEquals(expected, result);
+    }
+
+    @Test
+    public void testMergeIntervalsWithIdenticalIntervals() {
+        // Test case where multiple identical intervals are given
+        int[][] intervals = { { 1, 3 }, { 1, 3 }, { 1, 3 } };
+        int[][] expected = { { 1, 3 } };
+        int[][] result = MergeIntervals.merge(intervals);
+        assertArrayEquals(expected, result);
+    }
+
+    @Test
+    public void testMergeIntervalsWithRandomIntervals() {
+        // Test case with a mix of overlapping and non-overlapping intervals
+        int[][] intervals = { { 1, 4 }, { 5, 7 }, { 2, 6 }, { 8, 10 } };
+        int[][] expected = { { 1, 6 }, { 5, 7 }, { 8, 10 } };
+        int[][] result = MergeIntervals.merge(intervals);
+        assertArrayEquals(expected, result);
+    }
+}