[英]What's wrong with my merge sort implementation?
// merge operation for merge sort
private static void merge(int[] a, int left, int middle, int right) {
int[] temp = new int[right - left + 1];
int leftCrawler = left, rightCrawler = middle;
int currentIndex = 0;
while (leftCrawler < middle && rightCrawler <= right) {
if (a[leftCrawler] < a[rightCrawler])
temp[currentIndex++] = a[leftCrawler++];
else
temp[currentIndex++] = a[rightCrawler++];
}
while (leftCrawler < middle)
temp[currentIndex++] = a[leftCrawler++];
while (rightCrawler <= right)
temp[currentIndex++] = a[rightCrawler++];
// copy temp into a
for (int i = 0; i < temp.length; i++)
a[i] = temp[i];
}
private static void mergeSort(int[] a, int left, int right) {
if (right > left) {
int middle = left + (right - left) / 2;
mergeSort(a, left, middle);
mergeSort(a, middle + 1, right);
merge(a, left, middle, right);
}
}
public static void mergeSort(int[] a) {
int left = 0, right = a.length - 1;
mergeSort(a, left, right);
}
所以,我认为问题可能出在我的合并操作上,但是我在以下数组int[] a = {2, 5, 7, 15, 8, 9, 10}
, left = 0
, middle = 4
和right = a.length - 1
并且合并操作完成它成功所需的操作。
我将我的 mergeSort 实现与各种网站上的实现进行了比较,但我看不出有什么不同。 我的 mergeSort 不会成功地对数组进行排序。 我究竟做错了什么?
问题可能出在您的副本上:
// copy temp into a
for(int i = 0; i < temp.length; i++)
a[i] = temp[i];
你可能想要的是(注意left + i
):
// copy temp into a
for(int i = 0; i < temp.length; i++)
a[left + i] = temp[i];
(您的测试未检测到问题,因为left
为 0。)
您的代码中有 3 个错误:
合并循环不包括a[middle]
处的元素,因为您使用leftCrawler < middle
而不是:
while (leftCrawler <= middle && rightCrawler <= right)
第二个循环while (leftCrawler < middle)
也必须改为:
while (leftCrawler <= middle)
从temp
复制回a
的循环使用了不正确的索引到a
。 它应该是:
// copy temp into a for (int i = 0; i < temp.length; i++) a[left + i] = temp[i];
请注意,第一个错误源于此处使用的有害约定,其中right
和middle
包含在切片中而不是排除在外。 排除正确的边界允许更简单的代码而没有任何容易出错的+1
/ -1
调整。
这是一个修改后的版本:
// merge operation for merge sort
private static void merge(int[] a, int left, int middle, int right) {
int[] temp = new int[right - left];
int leftCrawler = left, rightCrawler = middle;
int currentIndex = 0;
while (leftCrawler < middle && rightCrawler < right) {
if (a[leftCrawler] < a[rightCrawler])
temp[currentIndex++] = a[leftCrawler++];
else
temp[currentIndex++] = a[rightCrawler++];
}
while (leftCrawler < middle)
temp[currentIndex++] = a[leftCrawler++];
while (rightCrawler < right)
temp[currentIndex++] = a[rightCrawler++];
// copy temp into a
for (int i = 0; i < temp.length; i++)
a[left + i] = temp[i];
}
private static void mergeSort(int[] a, int left, int right) {
if (right - left >= 2) {
int middle = left + (right - left) / 2;
mergeSort(a, left, middle);
mergeSort(a, middle, right);
merge(a, left, middle, right);
}
}
public static void mergeSort(int[] a) {
mergeSort(a, 0, a.length);
}
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