public class HeapSort {
  public static void main(String[] args) {
    int[] arr = {3, 2, 1, 0, -1, -2, -3};
    System.out.println("Before heap:");
    printArray(arr);
    heapSort(arr);
    System.out.println("After heap sort:");
    printArray(arr);
  }

  public static void heapSort(int[] arr) {
    if (arr == null || arr.length <= 1) {
      return;
    }
    buildMaxHeap(arr); //构建最大堆
    for (int i = arr.length - 1; i >= 1; i--) {
      exchangeElements(arr, 0, i); //交换堆顶和第0位置元素
      maxHeap(arr, i, 0); //因为交换元素后，有可能违反堆的性质，所以沉降元素
    }
  }

  private static void buildMaxHeap(int[] arr) { //构建最大堆
    if (arr == null || arr.length <= 1) {
      return;
    }
    int half = arr.length / 2;
    for (int i = half; i >= 0; i--) {
      maxHeap(arr, arr.length, i);
    }
  }

  private static void maxHeap(int[] arr, int heapSize, int index) {
    int left = index * 2 + 1; //左子树上的元素
    int right = index * 2 + 2; //右子树上的元素
    int largest = index; //初始化最大元素
    if (left < heapSize && arr[left] > arr[index]) {
      largest = left;
    }
    if (right < heapSize && arr[right] > arr[largest]) {
      largest = right;
    }
    if (index != largest) { //判断根元素是否为最大元素
      exchangeElements(arr, index, largest);
      maxHeap(arr, heapSize, largest);
    }
  }

  public static void printArray(int[] arr) { //打印数组
    System.out.print("{");
    for (int i = 0; i < arr.length; i++) {
      System.out.print(arr[i]);
      if (i < arr.length - 1) {
        System.out.print(", ");
      }
    }
    System.out.println("}");
  }

  public static void exchangeElements(int[] arr, int index1, int index2) { //交换元素
    int temp = arr[index1];
    arr[index1] = arr[index2];
    arr[index2] = temp;
  }
}