Question

我一直在努力寻找问题所在。但每次我运行这个程序时，都会显示：

Exception in thread "main" java.lang.NullPointerException 
  at MergeSortTest.mergeSort(MergeSortTest.java:8)

有人可以帮我解决这个问题吗？

public class MergeSortTest
{
    private static int[] arr;

    public static void mergeSort(int[] array)
    {
        int start = 0;
        int end = arr.length - 1;
        mergeSort(start, end);
        array = arr;
    }

    public static void mergeSort(int start, int end)
    {
        if (start == end) {
            return;
        }

        int center = (start + end) / 2;
        mergeSort(start, center);
        mergeSort(center + 1, end);
        merge(start, center + 1, end);
    }

    public static void merge(int left, int right, int rightEnd)
    {
        int k = left;
        int leftEnd = right-1;
        int num = rightEnd - left + 1;

        int front[] = new int[leftEnd - left + 1];
        int back[] = new int[rightEnd - right + 1];

        for (int i = left; i < front.length; i++)
        {
            front[i] = arr[i];
        }

        for (int i = right; i < back.length; i++)
        {
            back[i] = arr[i];
        }

        int[] temp = new int[num];

        while (true)
        {
            if (front[left] <= back[right])
            {
                if (left < leftEnd)
                {
                    temp[k] = front[left];
                    k++;
                    left++;
                }

                if (left == leftEnd)
                {
                    temp[k] = front[left];
                    for (right = right; right < rightEnd + 1; right++)
                    {
                        k++;
                        temp[k] = back[right];
                    }
                    break;
                }
            }
            else 
            {
                if (right < rightEnd)
                {
                    temp[k] = back[right];
                    k++;
                    right++; 
                }

                if (right == rightEnd)
                {
                    temp[k] = back[left];
                    for(left = left; left < leftEnd+1; left++)
                    {
                        k++;
                        temp[k] = front[left];
                    }
                    break;
                }
            }
        }
    }

    public static void main(String []args)
    {
        int[] array = new int[10];
        initializeRandom(array);
        for(int i = 0; i < array.length; i++)
        {
            System.out.print(array[i] + " ");
        }

        System.out.println();

        mergeSort(array);

        for (int i = 0; i < array.length; i++)
        {
            System.out.print(array[i] + " ");
        }
    }

    public static void initializeRandom(int[] array)
    {
        for (int i = 0; i < array.length; i++)
        {
            array[i] = (int)(Math.random() * 10 + 1);
        }
    }
}

Answer 1

您没有使用您作为参数传递的数组，但使用未初始化的数组：

private static int[] arr;

// ...
int end = arr.length - 1;

在main方法中：

mergeSort(array);

您需要将arr传递给merge或重写您的逻辑。

MergeSort中的边界异常

1 个答案: