test: add comprehensive unit tests for 2D segment tree

Author: YassaTalaat80

src/test/java/com/thealgorithms/datastructures/trees/SegmentTree2DTest.java

@@ -0,0 +1,70 @@
+package com.thealgorithms.datastructures.trees;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import org.junit.jupiter.api.Test;
+
+public class SegmentTree2DTest {
+
+    @Test
+    void testInitialEmptyQueries() {
+        SegmentTree2D segmentTree = new SegmentTree2D(4, 4);
+        
+        // Initial tree should return 0 for any query
+        assertEquals(0, segmentTree.query(0, 4, 0, 4));
+        assertEquals(0, segmentTree.query(1, 3, 1, 3));
+    }
+
+    @Test
+    void testUpdateAndPointQuery() {
+        SegmentTree2D segmentTree = new SegmentTree2D(5, 5);
+        
+        segmentTree.update(2, 3, 10);
+        segmentTree.update(0, 0, 5);
+
+        // Querying single points [row, row+1) x [col, col+1)
+        assertEquals(10, segmentTree.query(2, 3, 3, 4));
+        assertEquals(5, segmentTree.query(0, 1, 0, 1));
+        
+        // Empty point should be 0
+        assertEquals(0, segmentTree.query(1, 2, 1, 2));
+    }
+
+    @Test
+    void testSubmatrixQuery() {
+        SegmentTree2D segmentTree = new SegmentTree2D(4, 4);
+        
+        // Matrix simulation:
+        // [1, 2, 0, 0]
+        // [3, 4, 0, 0]
+        // [0, 0, 0, 0]
+        // [0, 0, 0, 0]
+        segmentTree.update(0, 0, 1);
+        segmentTree.update(0, 1, 2);
+        segmentTree.update(1, 0, 3);
+        segmentTree.update(1, 1, 4);
+
+        // Top-left 2x2 sum: 1+2+3+4 = 10
+        assertEquals(10, segmentTree.query(0, 2, 0, 2));
+
+        // First row sum: 1+2 = 3
+        assertEquals(3, segmentTree.query(0, 1, 0, 4));
+
+        // Second column sum: 2+4 = 6
+        assertEquals(6, segmentTree.query(0, 4, 1, 2));
+    }
+
+    @Test
+    void testUpdateOverwriting() {
+        SegmentTree2D segmentTree = new SegmentTree2D(3, 3);
+        
+        segmentTree.update(1, 1, 5);
+        assertEquals(5, segmentTree.query(1, 2, 1, 2));
+
+        // Overwrite the same point
+        segmentTree.update(1, 1, 20);
+        assertEquals(20, segmentTree.query(1, 2, 1, 2));
+        
+        // Full matrix sum should just be this point
+        assertEquals(20, segmentTree.query(0, 3, 0, 3));
+    }
+}