Java MergeSort with Strings

Question

我的老师本周出去了，她给了我们这个合并排序代码供我们使用。它是为int []数组编写的，我们应该为String []数组创建一个。

这是她的代码：

public static void mergeSort(int[ ] a, int from, int to)
{  if (from == to) return;
  int mid = (from + to) / 2;
   // sort the first and the second half
  mergeSort(a, from, mid);
  mergeSort(a, mid + 1, to);
  merge(a, from, mid, to);     }// end mergeSort

public static void merge(int[ ] a, int from, int mid, int to)
{  int n = to - from + 1;         // size of the range to be merged
  int[ ] b = new int[n]; // merge both halves into a temporary array b 
  int i1 = from;         // next element to consider in the first range
  int i2 = mid + 1;      // next element to consider in the second range
  int j = 0;             // next open position in b

  // as long as neither i1 nor i2 past the end, move the smaller into b
  while (i1 <= mid && i2 <= to)
  {  if (a[i1] < a[i2])
     {  b[j] = a[i1];
        i1++;          }
     else
     {  b[j] = a[i2];
        i2++;           }
     j++;
  }

  // note that only one of the two while loops below is executed

  // copy any remaining entries of the first half
  while (i1 <= mid)
  {  b[j] = a[i1];
     i1++;
     j++;        }

  // copy any remaining entries of the second half
  while (i2 <= to)
  {  b[j] = a[i2];
     i2++;
     j++;      }

  // copy back from the temporary array
  for (j = 0; j < n; j++)
     a[from + j] = b[j];
}// end merge

现在这是我的尝试：

//merge sort
public static void mergeSort(String[] a, int from, int to)
{  
    if (from == to)
        return;
    int mid = (from + to) / 2;
    // sort the first and the second half
    mergeSort(a, from, mid);
    mergeSort(a, mid + 1, to);
    merge(a, from, mid, to);
}// end mergeSort
//work
public static void merge(String[] a, int from, int mid, int to)
{  
    int n = to - from + 1;       // size of the range to be merged
    String[]b = new String[n];   // merge both halves into a temporary array b 
    int i1 = from;               // next element to consider in the first range
    int i2 = mid + 1;            // next element to consider in the second range
    int j = 0;                   // next open position in b

    // as long as neither i1 nor i2 past the end, move the smaller into b
    while (i1 <= mid && i2 <= to)
    {  
        if (a[i1].compareTo(a[i2]) > 0)
        {  
            b[j] = a[i1];
            i1++;          
        }
        else
        {  
            b[j] = a[i2];
            i2++;           
        }
        j++;
    }

    // note that only one of the two while loops below is executed

    // copy any remaining entries of the first half
    while (i1 <= mid)
    {  
        b[j] = a[i1];
        i1++;
        j++;
    }

    // copy any remaining entries of the second half
    while (i2 <= to)
    {  
        b[j] = a[i2];
        i2++;
        j++;
    }

    // copy back from the temporary array
    for (j = 0; j < n; j++)
        a[from + j] = b[j];
}//end merge

我觉得她给我们的代码遗漏了她通常会在课堂上向我们解释的内容，但是因为她出去了，我迷失了该怎么做。任何帮助表示赞赏。谢谢！

Answer 1

一切正常有效 - 只有compareTo中的merge方向错误。这是一个工作示例

import java.util.Arrays;

public class MergeSort {

    public static void main(String[] args) {
        String[] values = {"foo", "bar", "alice", "bob", "celine", "david"};
        mergeSort(values, 0, values.length - 1);
        System.out.println("Result " + Arrays.toString(values));
    }

    public static void mergeSort(String[] a, int from, int to) {
        if (from == to) {
            return;
        }
        int mid = (from + to) / 2;
        // sort the first and the second half
        mergeSort(a, from, mid);
        mergeSort(a, mid + 1, to);
        merge(a, from, mid, to);
    }// end mergeSort
//work

    public static void merge(String[] a, int from, int mid, int to) {
        int n = to - from + 1;       // size of the range to be merged
        String[] b = new String[n];   // merge both halves into a temporary array b
        int i1 = from;               // next element to consider in the first range
        int i2 = mid + 1;            // next element to consider in the second range
        int j = 0;                   // next open position in b

        // as long as neither i1 nor i2 past the end, move the smaller into b
        while (i1 <= mid && i2 <= to) {
            if (a[i1].compareTo(a[i2]) < 0) {
                b[j] = a[i1];
                i1++;
            } else {
                b[j] = a[i2];
                i2++;
            }
            j++;
        }

        // note that only one of the two while loops below is executed
        // copy any remaining entries of the first half
        while (i1 <= mid) {
            b[j] = a[i1];
            i1++;
            j++;
        }

        // copy any remaining entries of the second half
        while (i2 <= to) {
            b[j] = a[i2];
            i2++;
            j++;
        }

        // copy back from the temporary array
        for (j = 0; j < n; j++) {
            a[from + j] = b[j];
        }
    }//end merge

}