[英]is there a reason why my merge sort does not work for array length 10
我正在設置合並排序來對我的數組進行排序。 目標是對任意長度的數組進行排序。
我嘗試查看mergesort函數,但沒有看到任何錯誤。 該排序適用於某些數組長度,無論是奇數還是偶數,但對於數組長度(例如10),我得到了一個超限異常。
import java.util.Arrays;
class MergeSort
{
// Merge two sorted sub-arrays A[from .. mid] and A[mid + 1 .. to]
public static void merge(int[] A, int[] temp, int from, int mid, int to)
{
int k = from, i = from, j = mid + 1;
// loop till there are elements in the left and right runs
while (i <= mid && j <= to) {
if (A[i] < A[j]) {
temp[k++] = A[i++];
} else {
temp[k++] = A[j++];
}
}
// Copy remaining elements
while (i <= mid) {
temp[k++] = A[i++];
}
// Don't need to copy second half
// copy back to the original array to reflect sorted order
for (i = from; i <= to; i++) {
A[i] = temp[i];
}
}
// Iteratively sort array A[low..high] using temporary array
public static void mergesort(int[] A)
{
int low = 0;
int high = A.length - 1;
// sort array A[] using temporary array temp
int[] temp = Arrays.copyOf(A, A.length);
// divide the array into blocks of size m
// m = [1, 2, 4, 8, 16...]
for (int m = 1; m <= high - low; m = 2*m)
{
// for m = 1, i = 0, 2, 4, 6, 8...
// for m = 2, i = 0, 4, 8, 12...
// for m = 4, i = 0, 8, 16...
// ...
for (int i = low; i < high; i += 2*m)
{
int from = i;
int mid = i + m - 1;
int to = Integer.min(i + 2 * m - 1, high);
merge(A, temp, from, mid, to);
}
}
}
// Iterative Implementation of Mergesort algorithm
public static void main(String[] args)
{
int[] A = { 5, 7, -9, 3, -4, 2, 8, 8, 10, 11 };
System.out.println("Original Array : " + Arrays.toString(A));
mergesort(A);
System.out.println("Modified Array : " + Arrays.toString(A));
}
}
您的中間計算錯誤。 有時您將其設置在區域范圍之外。
下面的更改通過防止中點超出范圍來修復算法,類似於您對防止中點超出范圍所做的操作。
更改int mid = i + m - 1;
到int mid = Math.min(i + m - 1, A.length - 1);
說明:如您的評論中所述,您正在檢查的區域切片正在增大。 所以這是數組的排序方式,何時發生越界錯誤,以及為什么在以2的冪為單位的大小上未發生此錯誤:
[ -9, 5, 7, 3, -4, 2, 8, 8, 10, 11 ] Array size
First pass: [] [] [] [] [] [] [] [] [] [] 1
Second: [ ] [ ] [ ] [ ] [ ] 2
Third: [ ] [ ] [ ERROR] 4
這是您的解決方法:
為您的中部添加了一個if語句,使其不會越過數組邊界
if(mid<high) {
merge(A, temp, from, mid, to);
}
完整的代碼:
// Merge two sorted sub-arrays A[from .. mid] and A[mid + 1 .. to]
public static void merge(int[] A, int[] temp, int from, int mid, int to)
{
int k = from, i = from, j = mid + 1;
// loop till there are elements in the left and right runs
while (i <= mid && j <= to) {
if (A[i] < A[j]) {
temp[k++] = A[i++];
} else {
temp[k++] = A[j++];
}
}
// Copy remaining elements
while (i <= mid) {
temp[k++] = A[i++];
}
// Don't need to copy second half
// copy back to the original array to reflect sorted order
for (i = from; i <= to; i++) {
A[i] = temp[i];
}
}
// Iteratively sort array A[low..high] using temporary array
public static void mergesort(int[] A)
{
int low = 0;
int high = A.length - 1;
// sort array A[] using temporary array temp
int[] temp = Arrays.copyOf(A, A.length);
//System.out.println("temp Array : " + Arrays.toString(temp));
// divide the array into blocks of size m
// m = [1, 2, 4, 8, 16...]
for (int m = 1; m <= high - low; m = 2*m)
{
// for m = 1, i = 0, 2, 4, 6, 8...
// for m = 2, i = 0, 4, 8, 12...
// for m = 4, i = 0, 8, 16...
// ...
for (int i = low; i < high; i += 2*m)
{
int from = i;
int mid = i + m - 1;
int to = Integer.min(i + 2 * m - 1, high);
if(mid<high) {
merge(A, temp, from, mid, to);
}
}
}
}
// Iterative Implementation of Mergesort algorithm
public static void main(String[] args)
{
int[] A = { 5, 7, -9, 3, -4, 2, 8, 8, 10, 11 };
System.out.println("Original Array : " + Arrays.toString(A));
mergesort(A);
System.out.println("Modified Array : " + Arrays.toString(A));
}
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