add HeapSort

Author: Hawstein

java/src/main/java/io/algocasts/HeapSort.java

@@ -0,0 +1,44 @@
+package io.algocasts;
+
+public class HeapSort {
+
+  private void swap(int[] arr, int i, int j) {
+    int tmp = arr[i];
+    arr[i] = arr[j];
+    arr[j] = tmp;
+  }
+
+  // Time: O(log(n))
+  private void siftDown(int[] arr, int i, int end) {
+    int parent = i, child = 2 * parent + 1;
+    while (child <= end) {
+      if (child+1 <= end && arr[child+1] > arr[child]) ++child;
+      if (arr[parent] >= arr[child]) break;
+      swap(arr, parent, child);
+      parent = child;
+      child = 2 * parent + 1;
+    }
+  }
+
+  // i 从 end/2 开始即可，因为在二叉堆中，更大的 i 是没有子节点的，没必要做 siftDown
+  // Time: O(n)
+  // Reference:
+  // * https://www.geeksforgeeks.org/time-complexity-of-building-a-heap/
+  // * https://www2.cs.sfu.ca/CourseCentral/307/petra/2009/SLN_2.pdf
+  private void buildMaxHeap(int[] arr, int end) {
+    for (int i = end/2; i >= 0; --i) {
+      siftDown(arr, i, end);
+    }
+  }
+
+  // Time: O(n*log(n)), Space: O(1)
+  public void sort(int[] arr) {
+    if (arr == null || arr.length == 0) return;
+    buildMaxHeap(arr, arr.length - 1);
+    for (int end = arr.length - 1; end > 0; --end) {
+      swap(arr, 0, end);
+      siftDown(arr, 0, end - 1);
+    }
+  }
+
+}

java/src/test/java/io/algocasts/HeapSortTest.java

@@ -0,0 +1,13 @@
+package io.algocasts;
+
+import org.junit.Test;
+
+public class HeapSortTest extends SortTest {
+
+  @Test
+  public void test() {
+    HeapSort h = new HeapSort();
+    test(h::sort);
+  }
+
+}