[英]Merge sort Java Algorithm
嘿,我的合并排序一直遇到问题,我知道在线上有很多信息,而且已经多次出现,但是我不知道发生了什么,无论我做什么,这都行不通,有些帮助将不胜感激谢谢
我的主要方法如下:
for (int i = 0; i < trials; i++) {
data = generateRandomArray(arraySize);
// You can use following line to print out data for debugging
System.out.println("The array is: " + getString(data));
// ADD YOUR CODE HERE TO SORT DATA AND CALCULATE EXECUTION TIME
// System.out.println("first index:" + data[0]);
// System.out.println("first index:" + data[arraySize-1]);
//System.out.println("hello " + SortArray.basicPartition(data,0,data.length-1));
SortArray.mergeSort(data, 0, data.length-1);
if (isSorted(data))
System.out.println(" passes - array is sorted");
else
System.out.println(" failed - array is not sorted");
public static <T extends Comparable<? super T>>
void mergeSort(T[] a, T[] tempArray, int first, int last){
if(first < last) // We have some work to do
{
int mid = first+(last-first)/2;
mergeSort(a, tempArray, first, mid);
mergeSort(a, tempArray, mid+1, last);
merge(a, tempArray, first, mid, last);
}
} // end mergeSort
/** Merge the entries in two contiguous sorted sublists
* @param a An array of Comparable objects.
* @param tempArray A temporary array used in the merge.
* @param first An integer >= 0 and < mid.
* @param mid An integer <= last.
* @param last An integer < a.length.
*/
public static <T extends Comparable<? super T>>
void merge(T[] a, T[] tempArray, int first, int mid, int last){
int firstIndex = first;
int FirstHalfEnd = mid -1;
while ((first <= FirstHalfEnd) && (mid <= last)) {
if (a[first].compareTo(a[mid]) <= 0) {
tempArray[firstIndex] = a[first]; // last to first
firstIndex++;
first++;
}
else {
tempArray[firstIndex] = a[mid];
FirstHalfEnd++;
mid++;
//System.out.println("out of bounds");
}
}
while (first <= FirstHalfEnd) {
tempArray[firstIndex] = a[first];
firstIndex++;
first++;
}
while(mid <= last){
tempArray[firstIndex] = a[mid];
firstIndex++;
mid++;
}
for(int i=0;i<(last-first+1);i++){
a[last] = tempArray[last];
last--;
//System.out.println(a[i]);
}
} // end merge
OUTPUT
The array is: [ 1 5 3 5 1 6 9 7 1 4 ]
failed - array is not sorted
The array is: [ 1 8 3 4 3 1 6 8 0 9 ]
failed - array is not sorted
The array is: [ 0 1 5 5 5 0 0 3 0 4 ]
failed - array is not sorted
The array is: [ 0 0 6 2 7 4 6 2 2 2 ]
failed - array is not sorted
The array is: [ 4 9 2 3 3 4 4 0 3 5 ]
failed - array is not sorted
我没有运行您的代码-缺少片段-但我在merge()
函数的第一个while
循环中发现了2个问题-请参阅添加的注释:
while ((first <= FirstHalfEnd) && (mid <= last)) {
// compareTo return a negative value if (a[first] < a[mid])
// Then I think your comment is wrong: the values are put in the
// temporary array in increasing order. It means you have to review
// the for loop that copies the values
// at the end.
if (a[first].compareTo(a[mid]) <= 0) {
tempArray[firstIndex] = a[first]; // last to first (No!)
firstIndex++;
first++;
}
else {
tempArray[firstIndex] = a[mid];
FirstHalfEnd++; // <= this a bug, should be firstIndex++
mid++;
//System.out.println("out of bounds");
}
}
编辑
由于tempArray
值按tempArray
,因此for
循环的复制应遵循以下条件:
for(int i = first; i <= last; ++){
a[i] = tempArray[i];
}
哪个可以简化(?)或通过优化
System.arraycopy(tempArray, first, a, first, (last-first+1));
这是一个替代实现。 它以降序排列:
public class MergeSort {
public static void merge(int[]a,int[] aux, int f, int m, int l) {
for (int k = f; k <= l; k++) {
aux[k] = a[k];
}
int i = f, j = m+1;
for (int k = f; k <= l; k++) {
if(i>m) a[k]=aux[j++];
else if (j>l) a[k]=aux[i++];
else if(aux[j] > aux[i]) a[k]=aux[j++];
else a[k]=aux[i++];
}
}
public static void sort(int[]a,int[] aux, int f, int l) {
if (l<=f) return;
int m = f + (l-f)/2;
sort(a, aux, f, m);
sort(a, aux, m+1, l);
merge(a, aux, f, m, l);
}
public static int[] sort(int[]a) {
int[] aux = new int[a.length];
sort(a, aux, 0, a.length-1);
return a;
}
}
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