java - Java 归并排序算法

Question

嘿，我的合并排序遇到了问题，我知道网上有很多信息，而且这个问题已经出现多次，但我不知道发生了什么，无论我做什么，这都行不通，一些帮助将不胜感激谢谢

我的主要方法如下所示:

for (int i = 0; i < trials; i++) {
        data = generateRandomArray(arraySize);

        // You can use following line to print out data for debugging
        System.out.println("The array is: " + getString(data));

        // ADD YOUR CODE HERE TO SORT DATA AND CALCULATE EXECUTION TIME

        // System.out.println("first index:" + data[0]);
        // System.out.println("first index:" + data[arraySize-1]);

        //System.out.println("hello  " + SortArray.basicPartition(data,0,data.length-1));
        SortArray.mergeSort(data, 0, data.length-1);

        if (isSorted(data))
            System.out.println("   passes -  array is sorted");
        else
            System.out.println("   failed -  array is not sorted");

<小时/>

public static <T extends Comparable<? super T>>
void mergeSort(T[] a, T[] tempArray, int first, int last){
    if(first < last)        // We have some work to do
    {
        int mid = first+(last-first)/2;
        mergeSort(a, tempArray, first, mid);
        mergeSort(a, tempArray, mid+1, last);
        merge(a, tempArray, first, mid, last);
    }
} // end mergeSort

/** Merge the entries in two contiguous sorted sublists 
 * @param a An array of Comparable objects.
 * @param tempArray A temporary array used in the merge.
 * @param first An integer >= 0 and < mid.
 * @param mid An integer  <= last.
 * @param last An integer  < a.length.
 */
public static <T extends Comparable<? super T>>
void merge(T[] a, T[] tempArray, int first, int mid, int last){

    int firstIndex = first;
    int FirstHalfEnd = mid -1;


    while ((first <= FirstHalfEnd) && (mid <= last)) {

        if (a[first].compareTo(a[mid]) <= 0) {

            tempArray[firstIndex] = a[first]; // last to first
            firstIndex++;
            first++;
        } 
        else {
            tempArray[firstIndex] = a[mid];
            FirstHalfEnd++;
            mid++;
            //System.out.println("out of bounds");
        }
    }

    while (first <= FirstHalfEnd) {
        tempArray[firstIndex] = a[first];
        firstIndex++;
        first++;

    }
    while(mid <= last){
        tempArray[firstIndex] = a[mid];
        firstIndex++;
        mid++;
    }
    for(int i=0;i<(last-first+1);i++){ 
        a[last] = tempArray[last];
        last--;
        //System.out.println(a[i]);
    }

} // end merge

<小时/>

输出

The array is: [ 1 5 3 5 1 6 9 7 1 4 ]
   failed -  array is not sorted
The array is: [ 1 8 3 4 3 1 6 8 0 9 ]
   failed -  array is not sorted
The array is: [ 0 1 5 5 5 0 0 3 0 4 ]
   failed -  array is not sorted
The array is: [ 0 0 6 2 7 4 6 2 2 2 ]
   failed -  array is not sorted
The array is: [ 4 9 2 3 3 4 4 0 3 5 ]
   failed -  array is not sorted

Answer 1

我还没有运行你的代码 - 有缺失的部分 - 但我在 merge() 函数的第一个 while 循环中发现了 2 个问题 - 请参阅添加评论:

while ((first <= FirstHalfEnd) && (mid <= last)) {

    // compareTo return a negative value if (a[first] < a[mid])
    // Then I think your comment is wrong: the values are put in the 
    // temporary array in increasing order. It means you have to review
    // the for loop that copies the values 
    // at the end.
    if (a[first].compareTo(a[mid]) <= 0) {

        tempArray[firstIndex] = a[first]; // last to first (No!)
        firstIndex++;
        first++;
    } 
    else {
        tempArray[firstIndex] = a[mid];
        FirstHalfEnd++; // <= this a bug, should be firstIndex++
        mid++;
        //System.out.println("out of bounds");
    }
}

编辑
由于 tempArray 中的值按递增顺序排列，因此复制 for 循环应该是这样的:

for(int i = first; i <= last; ++){ 
    a[i] = tempArray[i];
}

可以通过以下方式简化(？)或优化哪些内容

System.arraycopy(tempArray, first, a, first, (last-first+1));