java堆排序 大堆

//时间复杂度 nLogn

import java.util.Arrays;

public class HeapSort {

    public static void main(String[] arg){
        int[] array = {6, 1, 9, 3, 8, 2, 7, 5, 4, 6, 1};
        heapSort(array);
        System.out.println(Arrays.toString(array));
    }

    private static void heapSort(int[] array){
        if(array == null || array.length < 2){
            return;
        }

        buildMaxHeap(array);

        for (int i = array.length - 1; i >=0; i--) {
            int temp = array[i];
            array[i] = array[0];
            array[0] = temp;

            maxHeap(array,i,0);
        }


    }

    /**
     * 构建大堆
     * @param array
     */
    private static void buildMaxHeap(int[] array){
        if (array == null || array.length < 2)
            return;
        int startIndex = (array.length - 1)/2;
        for (int i = startIndex; i >= 0 ; i--) {
            maxHeap(array,array.length,i);
        }
    }

    /**
     * @param array 数组
     * @param size 要调整的长度
     * @param index 从index开始
     */
    private static void maxHeap(int[] array,int size,int index){
        if(array == null || array.length < 2){
            return;
        }

        int leftIndex = index * 2 + 1;
        int rightIndex = index *2 + 2;
        int maxIndex = index;

        if(leftIndex < size && array[leftIndex] > array[maxIndex]){
            maxIndex = leftIndex;
        }

        if(rightIndex < size && array[rightIndex] > array[maxIndex]){
            maxIndex = rightIndex;
        }

        if (maxIndex != index) {
            int temp = array[index];
            array[index] = array[maxIndex];
            array[maxIndex] = temp;
            maxHeap(array,size,maxIndex);
        }

    }
}