Add all 23 GoF design pattern implementations (2026-06-13)

03-behavioral/strategy/MergeSort.java

@@ -0,0 +1,30 @@
+package strategy;
+
+import java.util.Arrays;
+
+public class MergeSort implements SortStrategy {
+    @Override
+    public void sort(int[] data) {
+        System.out.println("  [MergeSort] O(n log n) — stable, good for large datasets");
+        mergeSort(data, 0, data.length - 1);
+    }
+
+    private void mergeSort(int[] a, int l, int r) {
+        if (l >= r) return;
+        int m = (l + r) / 2;
+        mergeSort(a, l, m); mergeSort(a, m + 1, r);
+        merge(a, l, m, r);
+    }
+
+    private void merge(int[] a, int l, int m, int r) {
+        int[] left = Arrays.copyOfRange(a, l, m + 1);
+        int[] right = Arrays.copyOfRange(a, m + 1, r + 1);
+        int i = 0, j = 0, k = l;
+        while (i < left.length && j < right.length)
+            a[k++] = left[i] <= right[j] ? left[i++] : right[j++];
+        while (i < left.length)  a[k++] = left[i++];
+        while (j < right.length) a[k++] = right[j++];
+    }
+
+    @Override public String getName() { return "MergeSort"; }
+}