Add heap sort

Author: icgw

Basic/Sort/heapsort.cpp

@@ -0,0 +1,63 @@
+/*
+ * heapsort.cpp
+ * Copyright (C) 2019 Guowei Chen <[email protected]>
+ *
+ * Distributed under terms of the MIT license.
+ */
+
+
+#include <iostream>
+
+void swap (int &a, int &b) {
+  int tmp = a;
+  a = b;
+  b = tmp;
+}
+
+// NOTO: insert i into max heap.
+void heapify(int * arr, int n, int i) {
+  int largest = i;
+  int l = 2 * i + 1, r = 2 * i + 2;
+
+  if (l < n && arr[l] > arr[largest]) {
+    largest = l;
+  }
+
+  if (r < n && arr[r] > arr[largest]) {
+    largest = r;
+  }
+
+  if (i != largest) {
+    swap(arr[i], arr[largest]);
+    heapify(arr, n, largest);
+  }
+}
+
+void build_max_heap(int * arr, int n) {
+  for (int i = n / 2 - 1; i >= 0; --i) {
+    heapify(arr, n, i);
+  }
+}
+
+void heapsort(int * arr, int n) {
+  build_max_heap(arr, n);
+
+  for (int i = n - 1; i >= 0; --i) {
+    swap(arr[0], arr[i]);
+    heapify(arr, i, 0);
+  }
+}
+
+int main()
+{
+  int arr[] = { 10, 20, 15, 17, 9, 21 };
+  int n = 6;
+
+  heapsort(arr, n);
+
+  for (int i = 0; i < n; ++i) {
+    std::cout << arr[i] << "\n";
+  }
+
+  return 0;
+}