Java选择排序思路详解

    //选择排序
    /**
     * 时间复杂度：O(N^2) 和数据 是否有序无关
     * 空间复杂度：O(1)
     * 稳定性：不稳定
     * @param array
     */
    public static void selectSort(int[] array){
        for (int i = 0; i < array.length; i++) {
            int minIndex = i;
            for (int j = i+1; j < array.length; j++) {
                if(array[j] < array[minIndex]){
                    minIndex = j;
                }
            }
            swap(array,i,minIndex);
        }
    }
    private static void swap(int[] array, int i, int minIndex) {
        int tmp = array[i];
        array[i] = array[minIndex];
        array[minIndex] = tmp;
    }

private static void swap(int[] array, int i, int minIndex) {
    int tmp = array[i];
    array[i] = array[minIndex];
    array[minIndex] = tmp;
}
public static void selectSort1(int[] array){
    int left = 0;
    int right = array.length-1;
    while(left < right){
        int minIndex = left;
        int maxIndex = left;
        for (int i = left+1; i <= right; i++) {
            if(array[i] < array[minIndex]){
                minIndex = i;
            }
            if(array[i] > array[maxIndex]){
                maxIndex = i;
            }
        }
        swap(array, left, minIndex);
        //确保最大值的位置，最大值正好是 left 下标
        //此时把最大值换到了minIndex下标
        if(maxIndex == 0){
            maxIndex = minIndex;
        }
        swap(array, right, maxIndex);
        left++;
        right--;
    }
}

public static void createHeap(int[] array){
    for (int parent = (array.length-1-1)/2; parent >= 0; parent--) {
        siftDown(array,parent,array.length);//向下调整，创建大根堆
    }
}
public static void siftDown(int[] array, int parent, int length) {
    int child = 2*parent+1;
    while(child < length){
        if(child + 1 < length && array[child] < array[child +1]){
            child++;
        }
        if(array[child] > array[parent]){
            swap(array, child, parent);
            parent = child;
            child = 2*parent+1;
        }else{
            break;
        }
    }

public static void heapSort(int[] array){
    createHeap(array);
    int end  = array.length-1;
    while(end > 0){
        swap(array, 0 ,end);
        siftDown(array, 0,end);
        end--;
    }
}
public static void createHeap(int[] array){
    for (int parent = (array.length-1-1)/2; parent >= 0; parent--) {
        siftDown(array,parent,array.length);//向下调整，创建大根堆
    }
}
public static void siftDown(int[] array, int parent, int length) {
    int child = 2*parent+1;
    while(child < length){
        if(child + 1 < length && array[child] < array[child +1]){
            child++;
        }
        if(array[child] > array[parent]){
            swap(array, child, parent);
            parent = child;
            child = 2*parent+1;
        }else{
            break;
        }
    }
}