feat: Add Prefix Sum category with 1D and 2D implementations (#7220)

Co-authored-by: Deniz Altunkapan <deniz.altunkapan@outlook.com>

src/main/java/com/thealgorithms/prefixsum/PrefixSum.java

@@ -0,0 +1,54 @@
+package com.thealgorithms.prefixsum;
+
+/**
+ * A class that implements the Prefix Sum algorithm.
+ *
+ * <p>Prefix Sum is a technique used to preprocess an array such that
+ * range sum queries can be answered in O(1) time.
+ * The preprocessing step takes O(N) time.
+ *
+ * <p>This implementation uses a long array for the prefix sums to prevent
+ * integer overflow when the sum of elements exceeds Integer.MAX_VALUE.
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Prefix_sum">Prefix Sum (Wikipedia)</a>
+ * @author Chahat Sandhu, <a href="https://github.com/singhc7">singhc7</a>
+ */
+public class PrefixSum {
+
+    private final long[] prefixSums;
+
+    /**
+     * Constructor to preprocess the input array.
+     *
+     * @param array The input integer array.
+     * @throws IllegalArgumentException if the array is null.
+     */
+    public PrefixSum(int[] array) {
+        if (array == null) {
+            throw new IllegalArgumentException("Input array cannot be null");
+        }
+        this.prefixSums = new long[array.length + 1];
+        this.prefixSums[0] = 0;
+
+        for (int i = 0; i < array.length; i++) {
+            // Automatically promotes int to long during addition
+            this.prefixSums[i + 1] = this.prefixSums[i] + array[i];
+        }
+    }
+
+    /**
+     * Calculates the sum of elements in the range [left, right].
+     * Indices are 0-based.
+     *
+     * @param left  The starting index (inclusive).
+     * @param right The ending index (inclusive).
+     * @return The sum of elements from index left to right as a long.
+     * @throws IndexOutOfBoundsException if indices are out of valid range.
+     */
+    public long sumRange(int left, int right) {
+        if (left < 0 || right >= prefixSums.length - 1 || left > right) {
+            throw new IndexOutOfBoundsException("Invalid range indices");
+        }
+        return prefixSums[right + 1] - prefixSums[left];
+    }
+}

src/main/java/com/thealgorithms/prefixsum/PrefixSum2D.java

@@ -0,0 +1,64 @@
+package com.thealgorithms.prefixsum;
+
+/**
+ * A class that implements the 2D Prefix Sum algorithm.
+ *
+ * <p>2D Prefix Sum is a technique used to preprocess a 2D matrix such that
+ * sub-matrix sum queries can be answered in O(1) time.
+ * The preprocessing step takes O(N*M) time.
+ *
+ * <p>This implementation uses a long array for the prefix sums to prevent
+ * integer overflow.
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Summed-area_table">Summed-area table (Wikipedia)</a>
+ * @author Chahat Sandhu, <a href="https://github.com/singhc7">singhc7</a>
+ */
+public class PrefixSum2D {
+
+    private final long[][] prefixSums;
+
+    /**
+     * Constructor to preprocess the input matrix.
+     *
+     * @param matrix The input integer matrix.
+     * @throws IllegalArgumentException if the matrix is null or empty.
+     */
+    public PrefixSum2D(int[][] matrix) {
+        if (matrix == null || matrix.length == 0 || matrix[0].length == 0) {
+            throw new IllegalArgumentException("Input matrix cannot be null or empty");
+        }
+
+        int rows = matrix.length;
+        int cols = matrix[0].length;
+        this.prefixSums = new long[rows + 1][cols + 1];
+
+        for (int i = 0; i < rows; i++) {
+            for (int j = 0; j < cols; j++) {
+                // P[i+1][j+1] = current + above + left - diagonal_overlap
+                this.prefixSums[i + 1][j + 1] = matrix[i][j] + this.prefixSums[i][j + 1] + this.prefixSums[i + 1][j] - this.prefixSums[i][j];
+            }
+        }
+    }
+
+    /**
+     * Calculates the sum of the sub-matrix defined by (row1, col1) to (row2, col2).
+     * Indices are 0-based.
+     *
+     * @param row1 Top row index.
+     * @param col1 Left column index.
+     * @param row2 Bottom row index.
+     * @param col2 Right column index.
+     * @return The sum of the sub-matrix.
+     * @throws IndexOutOfBoundsException if indices are invalid.
+     */
+    public long sumRegion(int row1, int col1, int row2, int col2) {
+        if (row1 < 0 || row2 >= prefixSums.length - 1 || row2 < row1) {
+            throw new IndexOutOfBoundsException("Invalid row indices");
+        }
+        if (col1 < 0 || col2 >= prefixSums[0].length - 1 || col2 < col1) {
+            throw new IndexOutOfBoundsException("Invalid column indices");
+        }
+
+        return prefixSums[row2 + 1][col2 + 1] - prefixSums[row1][col2 + 1] - prefixSums[row2 + 1][col1] + prefixSums[row1][col1];
+    }
+}

src/test/java/com/thealgorithms/prefixsum/PrefixSum2DTest.java

@@ -0,0 +1,92 @@
+package com.thealgorithms.prefixsum;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+class PrefixSum2DTest {
+
+    @Test
+    @DisplayName("Test basic 3x3 square matrix")
+    void testStandardSquare() {
+        int[][] matrix = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
+        PrefixSum2D ps = new PrefixSum2D(matrix);
+
+        // Sum of top-left 2x2: {1,2, 4,5} -> 12
+        assertEquals(12L, ps.sumRegion(0, 0, 1, 1));
+        // Sum of bottom-right 2x2: {5,6, 8,9} -> 28
+        assertEquals(28L, ps.sumRegion(1, 1, 2, 2));
+        // Full matrix -> 45
+        assertEquals(45L, ps.sumRegion(0, 0, 2, 2));
+    }
+
+    @Test
+    @DisplayName("Test rectangular matrix (more cols than rows)")
+    void testRectangularWide() {
+        int[][] matrix = {{1, 1, 1, 1}, {2, 2, 2, 2}};
+        PrefixSum2D ps = new PrefixSum2D(matrix);
+
+        // Sum of first 3 columns of both rows -> (1*3) + (2*3) = 9
+        assertEquals(9L, ps.sumRegion(0, 0, 1, 2));
+    }
+
+    @Test
+    @DisplayName("Test rectangular matrix (more rows than cols)")
+    void testRectangularTall() {
+        int[][] matrix = {{1}, {2}, {3}, {4}};
+        PrefixSum2D ps = new PrefixSum2D(matrix);
+
+        // Sum of middle two elements -> 2+3 = 5
+        assertEquals(5L, ps.sumRegion(1, 0, 2, 0));
+    }
+
+    @Test
+    @DisplayName("Test single element matrix")
+    void testSingleElement() {
+        int[][] matrix = {{100}};
+        PrefixSum2D ps = new PrefixSum2D(matrix);
+
+        assertEquals(100L, ps.sumRegion(0, 0, 0, 0));
+    }
+
+    @Test
+    @DisplayName("Test large numbers for overflow (Integer -> Long)")
+    void testLargeNumbers() {
+        // 2 billion. Two of these sum to > MAX_INT
+        int val = 2_000_000_000;
+        int[][] matrix = {{val, val}, {val, val}};
+        PrefixSum2D ps = new PrefixSum2D(matrix);
+
+        // 4 * 2B = 8 Billion
+        assertEquals(8_000_000_000L, ps.sumRegion(0, 0, 1, 1));
+    }
+
+    @Test
+    @DisplayName("Test invalid inputs")
+    void testInvalidInputs() {
+        assertThrows(IllegalArgumentException.class, () -> new PrefixSum2D(null));
+        assertThrows(IllegalArgumentException.class, () -> new PrefixSum2D(new int[][] {})); // empty
+        assertThrows(IllegalArgumentException.class, () -> new PrefixSum2D(new int[][] {{}})); // empty row
+    }
+
+    @Test
+    @DisplayName("Test invalid query ranges")
+    void testInvalidRanges() {
+        int[][] matrix = {{1, 2}, {3, 4}};
+        PrefixSum2D ps = new PrefixSum2D(matrix);
+
+        // Negative indices
+        assertThrows(IndexOutOfBoundsException.class, () -> ps.sumRegion(-1, 0, 0, 0));
+        assertThrows(IndexOutOfBoundsException.class, () -> ps.sumRegion(0, -1, 0, 0));
+
+        // Out of bounds
+        assertThrows(IndexOutOfBoundsException.class, () -> ps.sumRegion(0, 0, 2, 0)); // row2 too big
+        assertThrows(IndexOutOfBoundsException.class, () -> ps.sumRegion(0, 0, 0, 2)); // col2 too big
+
+        // Inverted ranges (start > end)
+        assertThrows(IndexOutOfBoundsException.class, () -> ps.sumRegion(1, 0, 0, 0)); // row1 > row2
+        assertThrows(IndexOutOfBoundsException.class, () -> ps.sumRegion(0, 1, 0, 0)); // col1 > col2
+    }
+}

src/test/java/com/thealgorithms/prefixsum/PrefixSumTest.java

@@ -0,0 +1,80 @@
+package com.thealgorithms.prefixsum;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+class PrefixSumTest {
+
+    @Test
+    @DisplayName("Test basic sum with positive integers")
+    void testStandardCase() {
+        int[] input = {1, 2, 3, 4, 5};
+        PrefixSum ps = new PrefixSum(input);
+
+        // Sum of range [0, 4] -> 15
+        assertEquals(15L, ps.sumRange(0, 4));
+
+        // Sum of range [1, 3] -> 9
+        assertEquals(9L, ps.sumRange(1, 3));
+    }
+
+    @Test
+    @DisplayName("Test array with negative numbers and zeros")
+    void testNegativeAndZeros() {
+        int[] input = {-2, 0, 3, -5, 2, -1};
+        PrefixSum ps = new PrefixSum(input);
+
+        assertEquals(1L, ps.sumRange(0, 2));
+        assertEquals(-1L, ps.sumRange(2, 5));
+        assertEquals(0L, ps.sumRange(1, 1));
+    }
+
+    @Test
+    @DisplayName("Test with large integers to verify overflow handling")
+    void testLargeNumbers() {
+        // Two values that fit in int, but their sum exceeds Integer.MAX_VALUE
+        // Integer.MAX_VALUE is approx 2.14 billion.
+        int val = 2_000_000_000;
+        int[] input = {val, val, val};
+        PrefixSum ps = new PrefixSum(input);
+
+        // Sum of three 2 billion values is 6 billion (fits in long, overflows int)
+        assertEquals(6_000_000_000L, ps.sumRange(0, 2));
+    }
+
+    @Test
+    @DisplayName("Test single element array")
+    void testSingleElement() {
+        int[] input = {42};
+        PrefixSum ps = new PrefixSum(input);
+        assertEquals(42L, ps.sumRange(0, 0));
+    }
+
+    @Test
+    @DisplayName("Test constructor with null input")
+    void testNullInput() {
+        assertThrows(IllegalArgumentException.class, () -> new PrefixSum(null));
+    }
+
+    @Test
+    @DisplayName("Test empty array behavior")
+    void testEmptyArray() {
+        int[] input = {};
+        PrefixSum ps = new PrefixSum(input);
+        assertThrows(IndexOutOfBoundsException.class, () -> ps.sumRange(0, 0));
+    }
+
+    @Test
+    @DisplayName("Test invalid range indices")
+    void testInvalidIndices() {
+        int[] input = {10, 20, 30};
+        PrefixSum ps = new PrefixSum(input);
+
+        assertThrows(IndexOutOfBoundsException.class, () -> ps.sumRange(-1, 1));
+        assertThrows(IndexOutOfBoundsException.class, () -> ps.sumRange(0, 3));
+        assertThrows(IndexOutOfBoundsException.class, () -> ps.sumRange(2, 1));
+    }
+}