基于什么是有效的排序算法？

Question

我已经实现了一种排序算法来对无序整数数组进行排序。如果我投入一个100,000个元素的阵列，它需要大约10,500毫秒才能返回。我的算法是快还是慢？以下是代码。

public static void sort(int[] array) {
    int lastItem = array[array.length - 1];
    int length = array.length;
    for (int j = 0; j < array.length; j++) {

        for (int i = 0; i < length; i++) {

            if (array[i] > lastItem) {
                array[length-1] = array[i];
                array[i] = lastItem;
                lastItem = array[length - 1];
            }
        }
        length--;
        if (length > 1) lastItem = array[length - 1];
    }
}

Answer 1

你的算法使用冒泡排序，它取o（n ^ 2）。对于较大的输入，它可能很慢。为什么不使用快速排序来实现你想要的结果在O（nlogn）？

以下是一些代码，请注意，选择枢轴作为中间元素可能更好。

/**
 * o(nlogn) - high probability otherwise o(n SQUARE)
 * 
 * 
 * Choose a pivot value. We take the value of the middle element 
 * as pivot value, but it can be any value, which is in range of 
 * sorted values, even if it doesn't present in the array.
 * 
 * Partition. Rearrange elements in such a way, that all elements 
 * which are lesser than the pivot go to the left part of the array 
 * and all elements greater than the pivot, go to the right part 
 * of the array. Values equal to the pivot can stay in any part 
 * of the array. Notice, that array may be divided in non-equal parts.
 * 
 * Sort both parts. Apply quicksort algorithm recursively to the left 
 * and the right parts.
 * 
 * @param input
 */
public void quickSort(int[] input, int start, int end){
    if( start < end ){
        int pindex = findParition(input, start, end);
        quickSort(input, start, pindex-1);
        quickSort(input, pindex+1, end);
    }
}

/**
 * findParition for quick sort
 * @param input
 * @param start
 * @param end
 * @return
 */
private int findParition(int[] input, int start, int end) {
    int pivot = input[end];
    int pindex = start;

    for( int i = start; i < end; i++){
        if( input[i] <= pivot ){
            int temp = input[pindex];
            input[pindex] = input[i];
            input[i] = temp;

            pindex++;
        }
    }

    int temp = input[pindex];
    input[pindex] = input[end];
    input[end] = temp;

    return pindex;
}

Answer 2

Click here比较各种种类。