排序 堆排序

public class HeapSort {

    static class ArrayUtils {

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

        public static void exchangeElements(int[] array, int index1, int index2) {
            int temp = array[index1];
            array[index1] = array[index2];
            array[index2] = temp;
        }
    }

    public static void main(String[] args) {
        int[] array = { 9, 8, 13, 7, 6, 5, 4, 3, 2, 1, 0, -1, -2, -3, 12, 11, 15, 14 };

        System.out.println("Before heap:");
        ArrayUtils.printArray(array);

        heapSort(array);

        System.out.println("After heap sort:");
        ArrayUtils.printArray(array);
    }

    public static void heapSort(int[] array) {
//      if (array == null || array.length <= 1) {
//          return;
//      }
        buildMaxHeap(array);
        for (int i = array.length - 1; i >= 1; i--) {
            ArrayUtils.exchangeElements(array, 0, i);
            maxHeap(array, i, 0);
        }
    }

    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--) {
            maxHeap(array, array.length, i);
        }
    }

    // 堆調整，使其生成最大堆
    private static void maxHeap(int[] array, int heapSize, int index) {
        int left = index * 2 + 1;// 左子節點索引
        int right = index * 2 + 2;// 右子節點索引
        int largest = index;// 最大節點索引
        // 如果左子節點大於父節點，則將左子節點作爲最大節點
        if (left < heapSize && array[left] > array[index]) {
            largest = left;
        }
        // 如果右子節點比最大節點還大，那麼最大節點應該是右子節點
        if (right < heapSize && array[right] > array[largest]) {
            largest = right;
        }
        // 最後，如果最大節點和父節點不一致，則交換他們的值
        if (index != largest) {
            ArrayUtils.exchangeElements(array, index, largest);
            // 交換完父節點和子節點的值，對換了值的子節點檢查是否符合最大堆的特性
            maxHeap(array, heapSize, largest);
        }
        ArrayUtils.printArray(array);
    }

    public static void maxHeaps(int[] array,int heapSize,int index){
        if(array==null || array.length<=1){
            return;
        }
        int left = 2*index+1;
        int right = 2*index+2;
        int largest = index;
        if(left<heapSize && array[left]>array[index]){
            largest=left;
        }
        if(right<heapSize && array[right]>array[largest]){
            largest=right;
        }
        if(index!=largest){
            ArrayUtils.exchangeElements(array, index, largest);
            maxHeaps(array, heapSize, largest);
        }

    }
}