Spiral Matrix II

src/main/java/com/thealgorithms/matrix/SpiralMatrixII.java

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+/**
+ * Spiral Matrix II
+ * Generates an n x n matrix filled with numbers from 1 to n^2 in spiral order.
+ *
+ * @see <a href="https://leetcode.com/problems/spiral-matrix-ii/">LeetCode – Spiral Matrix II</a>
+ * @see <a href="https://en.wikipedia.org/wiki/Spiral_matrix">Wikipedia – Spiral Matrix</a>
+ */
+package com.thealgorithms.matrix;
+
+public class SpiralMatrixII {
+
+    public int[][] generateMatrix(int n) {
+
+        int[][] matrix = new int[n][n];
+
+        int top = 0;
+        int bottom = n - 1;
+
+        int left = 0;
+        int right = n - 1;
+
+        int num = 1;
+
+        while (top <= bottom && left <= right) {
+
+            for (int i = left; i <= right; i++) {
+                matrix[top][i] = num++;
+            }
+            top++;
+
+            for (int i = top; i <= bottom; i++) {
+                matrix[i][right] = num++;
+            }
+            right--;
+
+            if (top <= bottom) {
+                for (int i = right; i >= left; i--) {
+                    matrix[bottom][i] = num++;
+                }
+                bottom--;
+            }
+
+            if (left <= right) {
+                for (int i = bottom; i >= top; i--) {
+                    matrix[i][left] = num++;
+                }
+                left++;
+            }
+        }
+
+        return matrix;
+    }
+}

src/test/java/com/thealgorithms/matrix/SpiralMatrixIITest.java

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+package com.thealgorithms.matrix;
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import org.junit.jupiter.api.Test;
+
+class SpiralMatrixTest {
+
+    SpiralMatrix spiral = new SpiralMatrix();
+
+    @Test
+    void testNEquals3() {
+        int[][] expected = {{1, 2, 3}, {8, 9, 4}, {7, 6, 5}};
+        assertArrayEquals(expected, spiral.generateMatrix(3));
+    }
+
+    @Test
+    void testNEquals4() {
+        int[][] expected = {{1, 2, 3, 4}, {12, 13, 14, 5}, {11, 16, 15, 6}, {10, 9, 8, 7}};
+        assertArrayEquals(expected, spiral.generateMatrix(4));
+    }
+
+    @Test
+    void testNEquals2() {
+        int[][] expected = {{1, 2}, {4, 3}};
+        assertArrayEquals(expected, spiral.generateMatrix(2));
+    }
+}