restructure and add ipi texs

ws2019/ipi/uebungen/cpp_headers/Mergesort.cc

@@ -0,0 +1,29 @@
+template <class C> 
+void rec_merge_sort (C& a, int o, int n) 
+{   // sortiere Eintraege [o,o+n-1]
+  if (n==1) return;
+    
+  // teile und sortiere rekursiv
+  int n1=n/2;
+  int n2=n-n1;
+  rec_merge_sort(a,o,n1);
+  rec_merge_sort(a,o+n1,n2);
+
+  // zusammenfuegen
+  C b(n); // Hilfsfeld
+  int i1=o, i2=o+n1;
+  for (int k=0; k<n; k=k+1)
+    if ((i2>=o+n) || (i1<o+n1 && a[i1]<=a[i2]))
+      b[k] = a[i1++];
+    else
+      b[k] = a[i2++]; 
+
+  // umkopieren
+  for (int k=0; k<n; k=k+1) a[o+k] = b[k];
+}
+
+template <class C> 
+void mergesort (C& a) 
+{
+  rec_merge_sort(a,0,a.size());
+}