递归合并排序不返回已排序数组

Question

我写了3个方法来实现递归合并排序，参数数量有限（没有aux、lo、mid、hi)。我认为我的工作是这样的，但它并没有返回一个排序数组，尽管它在运行时没有任何编译错误。我已经摆弄了4个小时，似乎无法弄清楚我做错了什么，没有合并一个有序数组。我只从我的助教那里得到了非常模糊的输入，并且能够修复我正在遇到的一些问题，但是该方法仍然没有对项数组进行排序。欢迎任何关于我在这里做错了什么的建议。谢谢！

    public static void sort(Comparable[] a) {
        //Sort a recursively
        a = mergeSort(a);

    }
    
    
    /** 
     * 
     * Method recursively splits array into halves until length becomes 1, then
     * Calls merge to merge the split arrays back together.
     * @param a Comparable[] 
     * @return a Comparable[]
     */
    public static Comparable[] mergeSort(Comparable[] a) {
        if (a.length <= 1) {
            return a;
        }
        else {
        //create 1st array for D&C, length = 1/2 a.length
        Comparable[] first = new Comparable[a.length / 2]; 
        
        //create 2nd array for D&C, length = a.length - first.length
        Comparable[] second = new Comparable[a.length - first.length]; 
        
      //source/position: a[0], destination/position: first[0], length: first.length
        System.arraycopy(a, 0, first, 0, first.length);      
        
      //source/position: a[first.length], destination/pos: second[0], length: second.length
        System.arraycopy(a, first.length, second, 0, second.length);  
        
        //recursively sort the first half
        first = mergeSort(first);
        
        //recursively sort second half
        second =  mergeSort(second);
        
        //merge the halves
        a= merge(first,second);
        }
        
        //return the merged array
        return a;
    }
    
    public static Comparable[] merge(Comparable[] a, Comparable[] b) {
        Comparable[] result = new Comparable[a.length + b.length];
        
            //Index Position in first array - starting with first element
            int first = 0;
             
            //Index Position in second array - starting with first element
            int second = 0;
             
            //Index Position in merged array - starting with first position
            int merged = 0;
             
            //Compare first and second, 

            while (first < a.length && second < b.length) 
            {
                if (a[first].compareTo(b[second]) < 0) 
                {
                    result[merged++] = a[first++];
  
                } 
                else
                {
                    result[merged++] = b[second++];
             
                }
             
            }
            
            //Store remaining elements of 1st array
            while(first < a.length) {
                result[merged++] = a[first++];
            }
            
            //Store remaining elements of second array
            while(second < b.length) {
                result[merged++] = b[second++];
            }
            
            //copy elements from both halves - each half will have the sorted elements
           // System.arraycopy(a, first, result, merged, a.length - first);
           // System.arraycopy(b, second, result, merged, b.length - second);
            
            return result;
        }
    

Answer 1

正如@Thomas所指出的，Java函数是按值传递的。因此，执行a=mergeSort(a)不会更改原始数组，而是对传递给function的数组的副本进行排序。相反，您可以返回结果：

public static Comperable[] sort(Comparable[] a) {
        //Sort a recursively
        return mergeSort(a);
}

这将用本地值计算所有东西，并将其归还给原始“所有者”。