restructure and add ipi texs

ws2019/ipi/uebungen/cpp_headers/Heapsort.cc

@@ -0,0 +1,25 @@
+template <class C>
+inline void reheap (C& a, int n, int i) {
+  while (2*i+1<n)
+    {
+      int l = 2*i+1;
+      int r = l+1;
+      int k = ((r<n) && (a[r]>a[l])) ? r : l;
+      if (a[k]<=a[i]) break;
+      std::swap(a[k], a[i]);
+      i = k;
+    }
+}
+
+template <class C> 
+void heapsort (C& a)
+{
+  // build the heap by reheaping from the rear
+  for (int i=a.size()-1; i>=0; i--)
+    reheap(a, a.size(), i);
+  // build the sorted list by popping the heap 
+  for (int i=a.size()-1; i>=0; i--) {
+    std::swap(a[0],a[i]);
+    reheap(a, i, 0);
+  }
+}