排序之堆排序

public class HeapSort {
    public static void main(String[] args) {
        CreateNoOrderArray createNoOrderArray = new CreateNoOrderArray();
        int[] array = createNoOrderArray.creat();
        System.out.println("Before heap:");
        System.out.println(Arrays.toString(array));
        heapSort(array);
        System.out.println("After heap sort:");
        System.out.println(Arrays.toString(array));
    }

    /**
     * 堆排序
     * @param array
     */
    private static void heapSort(int[] array) {
        if (array == null || array.length <= 1) {
            return;
        }
        buildMaxHeap(array);
        for (int i = array.length - 1; i >= 1; i--) {
            swap(array, 0, i);   //每次把最大的换到最后
            heapify(array, i, 0);  //重新把次大的调整到根节点
        }
    }

    /**
     * 构造最大堆
     * @param array
     */
    private static void buildMaxHeap(int[] array) {
        if (array == null || array.length <= 1) {
            return;
        }
        int half = array.length / 2;
        for (int i = half; i >= 0; i--) {   //从最后一个节点的父节点开始调整，使根节点是最大的
            heapify(array, array.length, i);
        }
    }

    /**
     * 从index向下调整
     * @param array
     * @param heapSize
     * @param index
     */
    private static void heapify(int[] array, int heapSize, int index) {
        int left = index * 2 + 1;
        int right = index * 2 + 2;
        int largest = index;
        while(left < heapSize){
            if (array[left] > array[index]) {
                largest = left;
            }

            if (right < heapSize && array[right] > array[largest]) {
                largest = right;
            }

            if (index != largest) {
                swap(array, index, largest);
            }else {
                break;
            }
            index = largest;
            left = index * 2 + 1;
            right = index * 2 + 2;
        }

    }
    private static void swap(int[] array, int i, int j) {
        int temp = array[i];
        array[i] = array[j];
        array[j] = temp;
    }
}