问题
I have a problem with implementation of merge sort in java. I am looking for the error almost week unfortunately without result. ArrayList at the entrance is the same as the output.
import java.util.ArrayList;
import java.util.Scanner;
public class MergeSort
{
private ArrayList<Integer> basicArrayList = new ArrayList<Integer>();
ArrayList<Integer> arrayListA = new ArrayList<Integer>();
ArrayList<Integer> arrayListB = new ArrayList<Integer>();
Scanner input = new Scanner(System.in);
private int firstIndexOfArrayList = 0;
private int lastIndexOfArrayListA;
private int lastIndexOfArrayListB;
public void Scal(ArrayList<Integer> basicArrayList, int p, int q, int r) {
this.firstIndexOfArrayList = p;
this.lastIndexOfArrayListA = q;
this.lastIndexOfArrayListB = r;
int numberOfElementsArrayListA = lastIndexOfArrayListA
- firstIndexOfArrayList + 1;
int numberOfElementsArrayListB = lastIndexOfArrayListB
- lastIndexOfArrayListA;
for (int i = 0; i < numberOfElementsArrayListA; i++) {
arrayListA.set(i, basicArrayList.get(firstIndexOfArrayList + i));
}
for (int j = 0; j < numberOfElementsArrayListB; j++) {
arrayListB.set(j, basicArrayList.get(lastIndexOfArrayListA + j));
}
arrayListA.add(Integer.MAX_VALUE);
arrayListB.add(Integer.MAX_VALUE);
int i = 0;
int j = 0;
for (int k = firstIndexOfArrayList; k <= lastIndexOfArrayListB; k++) {
if (arrayListA.get(i) <= arrayListB.get(j)) {
basicArrayList.set(k, arrayListA.get(i));
i = i + 1;
} else {
basicArrayList.set(k, arrayListB.get(j));
j = j + 1;
}
}
}
public void MergeSort(ArrayList basicArrayList, int p, int r) {
this.firstIndexOfArrayList = p;
this.lastIndexOfArrayListB = r;
if (firstIndexOfArrayList < lastIndexOfArrayListB) {
int lastIndexOfArrayListA = (firstIndexOfArrayList + lastIndexOfArrayListB) / 2;
MergeSort(basicArrayList, firstIndexOfArrayList,
lastIndexOfArrayListA);
MergeSort(basicArrayList, lastIndexOfArrayListA + 1,
lastIndexOfArrayListB);
Scal(basicArrayList, firstIndexOfArrayList,
lastIndexOfArrayListA,
lastIndexOfArrayListB);
}
}
public void setSize() {
System.out.println("Enter the number of elements to sort: ");
this.lastIndexOfArrayListB = input.nextInt();
}
public int getSize() {
return lastIndexOfArrayListB;
}
public void setData() {
System.out.println("Enter the numbers: ");
for (int i = 0; i < lastIndexOfArrayListB; i++) {
int number;
number = input.nextInt();
basicArrayList.add(number);
}
}
public void getTable() {
System.out.println(basicArrayList.toString());
}
public static void main(String[] args) {
MergeSort output = new MergeSort();
output.setSize();
output.setData();
output.MergeSort(output.basicArrayList,
output.firstIndexOfArrayList, (output.getSize() - 1));
output.getTable();
}
}
回答1:
In terms of fixing your code I had a crack at it and as far as I can tell this seems to work. To do this a lot of your code had to be changed but it does now sort all Integers properly
import java.util.ArrayList;
import java.util.Scanner;
public class MergeSort
{
private ArrayList<Integer> basicArrayList = new ArrayList<Integer>();
Scanner input = new Scanner(System.in);
private int numbersToSort;
public void doMergeSort(int firstIndexOfArrayList,int lastIndexOfArrayListB, ArrayList<Integer> arrayList)
{
if(firstIndexOfArrayList<lastIndexOfArrayListB && (lastIndexOfArrayListB-firstIndexOfArrayList)>=1)
{
int mid = (lastIndexOfArrayListB + firstIndexOfArrayList)/2;
doMergeSort(firstIndexOfArrayList, mid, arrayList);
doMergeSort(mid+1, lastIndexOfArrayListB, arrayList);
Scal(firstIndexOfArrayList,mid,lastIndexOfArrayListB, arrayList);
}
}
public void Scal(int firstIndexOfArrayList,int lastIndexOfArrayListA,int lastIndexOfArrayListB, ArrayList<Integer> arrayList)
{
ArrayList<Integer> mergedSortedArray = new ArrayList<Integer>();
int leftIndex = firstIndexOfArrayList;
int rightIndex = lastIndexOfArrayListA+1;
while(leftIndex<=lastIndexOfArrayListA && rightIndex<=lastIndexOfArrayListB)
{
if(arrayList.get(leftIndex)<=arrayList.get(rightIndex))
{
mergedSortedArray.add(arrayList.get(leftIndex));
leftIndex++;
}
else
{
mergedSortedArray.add(arrayList.get(rightIndex));
rightIndex++;
}
}
while(leftIndex<=lastIndexOfArrayListA)
{
mergedSortedArray.add(arrayList.get(leftIndex));
leftIndex++;
}
while(rightIndex<=lastIndexOfArrayListB)
{
mergedSortedArray.add(arrayList.get(rightIndex));
rightIndex++;
}
int i = 0;
int j = firstIndexOfArrayList;
while(i<mergedSortedArray.size())
{
arrayList.set(j, mergedSortedArray.get(i++));
j++;
}
}
public void setSize()
{
System.out.println("Enter the number of elements to sort: ");
this.numbersToSort = input.nextInt();
}
public int getSize()
{
return numbersToSort;
}
public void setData()
{
System.out.println("Enter the numbers: ");
for (int i = 0; i < numbersToSort; i++)
{
int number;
number = input.nextInt();
basicArrayList.add(number);
}
}
public void getTable()
{
System.out.println(basicArrayList.toString());
}
public void runSort(ArrayList<Integer> arrayList)
{
doMergeSort(0, this.numbersToSort-1, arrayList);
}
public static void main(String[] args)
{
MergeSort output = new MergeSort();
output.setSize();
output.setData();
output.runSort(output.basicArrayList);
output.getTable();
}
}
回答2:
Try this code. The following code takes an ArrayList input and outputs an ArrayList as well so it still works along the same basis of your code. The actual sort is handled in a different class MergeSort and is passes into ForMergeSort. Hope this helps
MergeSort.java
public class MergeSort
{
private int[] array;
private int[] tempMergArr;
private int length;
public void sort(int[] inputArr)
{
}
public int[] getSortedArray(int[] inputArr)
{
this.array = inputArr;
this.length = inputArr.length;
this.tempMergArr = new int[length];
doMergeSort(0, length - 1);
for(int i=0;i<length;i++)
{
int correctNumber = i+1;
System.out.println("Value "+correctNumber+" of the sorted array which was sorted via the Merge Sort is: "+inputArr[i]);
}
return inputArr;
}
private void doMergeSort(int lowerIndex, int higherIndex)
{
if (lowerIndex < higherIndex)
{
int middle = lowerIndex + (higherIndex - lowerIndex) / 2;
doMergeSort(lowerIndex, middle);
doMergeSort(middle + 1, higherIndex);
mergeParts(lowerIndex, middle, higherIndex);
}
}
private void mergeParts(int lowerIndex, int middle, int higherIndex)
{
for (int i = lowerIndex; i <= higherIndex; i++)
{
tempMergArr[i] = array[i];
}
int i = lowerIndex;
int j = middle + 1;
int k = lowerIndex;
while (i <= middle && j <= higherIndex)
{
if (tempMergArr[i] <= tempMergArr[j])
{
array[k] = tempMergArr[i];
i++;
}
else
{
array[k] = tempMergArr[j];
j++;
}
k++;
}
while (i <= middle)
{
array[k] = tempMergArr[i];
k++;
i++;
}
}
}
ForMergeSort.java
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Scanner;
public class ForMergeSort
{
ArrayList<Integer> arrayList = new ArrayList<Integer>();
ArrayList<Integer> sortedArrayList = new ArrayList<Integer>();
MergeSort mS = new MergeSort();
public void buildArrayList()
{
Scanner input = new Scanner(System.in);
System.out.println("Enter the number of elements to sort: ");
int toSort = input.nextInt();
System.out.println("Enter the numbers: ");
for(int i =0; i<toSort; i++)
{
int number = input.nextInt();
arrayList.add(number);
}
}
public void runMergeSort(ArrayList<Integer> arrayList)
{
int[] arrayOfValues = new int[arrayList.size()];
int i = 0;
for(int a:arrayList)
{
arrayOfValues[i] = a;
i++;
}
MergeSort mS = new MergeSort();
for(int intOfArray:mS.getSortedArray(arrayOfValues))
{
sortedArrayList.add(intOfArray);
}
System.out.println(sortedArrayList.toString());
}
public static void main(String[] args)
{
ForMergeSort fMS = new ForMergeSort();
fMS.buildArrayList();
fMS.runMergeSort(fMS.arrayList);
}
}
来源：https://stackoverflow.com/questions/34375648/implementing-merge-sort-in-java-7