英文:
Java Top-Down Merge Sort - Stackoverflow Error
问题
以下是你要的翻译部分：
import java.util.ArrayList;
import java.util.Random;
public class Main {
public static void main(String[] args) {
Random r = new Random();
ArrayList<Integer> numbers = new ArrayList<Integer>();
for (int i = 1; i <= 15; i++) {
numbers.add(r.nextInt(100));
}
numbers = mergeSort(numbers);
System.out.println(numbers);
}
public static ArrayList<Integer> mergeSort(ArrayList<Integer> m) {
if (m.size() <= 1) {
return m;
}
ArrayList<Integer> left = new ArrayList<Integer>();
ArrayList<Integer> right = new ArrayList<Integer>();
for (Integer x : m) {
if (m.indexOf(x) < (m.size()) / 2)
left.add(x);
else {
right.add(x);
}
}
left = mergeSort(left);
right = mergeSort(right);
return merge(left, right);
}
private static ArrayList<Integer> merge(ArrayList<Integer> l, ArrayList<Integer> r) {
ArrayList<Integer> result = new ArrayList<Integer>();
while (l.size() > 0 && r.size() > 0) {
if (l.get(0) <= r.get(0)) {
result.add(l.get(0));
l.remove(0);
}
else {
result.add(r.get(0));
r.remove(0);
}
}
while (l.size() > 0) {
result.add(l.get(0));
l.remove(0);
}
while (r.size() > 0) {
result.add(r.get(0));
r.remove(0);
}
return result;
}
}
英文:
I am trying to implement the top-down merge sort algorithm in Java, using the pseudocode from Wikipedia.
My problem is that my code sometimes throws a StackOverflowError, but not always. I have checked that my code matches the pseudocode several times and cannot find what is wrong with it.
Here is my Java code:
import java.util.ArrayList;
import java.util.Random;
public class Main {
public static void main(String[] args) {
Random r = new Random();
ArrayList<Integer> numbers = new ArrayList<Integer>();
for (int i = 1; i <= 15; i++) {
numbers.add(r.nextInt(100));
}
numbers = mergeSort(numbers);
System.out.println(numbers);
}
public static ArrayList<Integer> mergeSort(ArrayList<Integer> m) {
if (m.size() <= 1) {
return m;
}
ArrayList<Integer> left = new ArrayList<Integer>();
ArrayList<Integer> right = new ArrayList<Integer>();
for (Integer x : m) {
if (m.indexOf(x) < (m.size()) / 2)
left.add(x);
else {
right.add(x);
}
}
left = mergeSort(left);
right = mergeSort(right);
return merge(left, right);
}
private static ArrayList<Integer> merge(ArrayList<Integer> l, ArrayList<Integer> r) {
ArrayList<Integer> result = new ArrayList<Integer>();
while (l.size() > 0 && r.size() > 0) {
if (l.get(0) <= r.get(0)) {
result.add(l.get(0));
l.remove(0);
}
else {
result.add(r.get(0));
r.remove(0);
}
}
while (l.size() > 0) {
result.add(l.get(0));
l.remove(0);
}
while (r.size() > 0) {
result.add(r.get(0));
r.remove(0);
}
return result;
}
}
答案1
得分: 2
你的算法在存在重复元素时会出问题，因为 indexOf 只会返回第一个元素的索引。改用基于索引的 for 循环。 <sup>示例</sup>
for (int i = 0; i < m.size(); i++) {
if (i < (m.size()) / 2)
left.add(m.get(i));
else {
right.add(m.get(i));
}
}
英文:
Your algorithm encounters issues when there are duplicate elements, as indexOf will only return the index of the first one. Use a index-based for loop instead. <sup>Demo</sup>
for (int i = 0; i < m.size(); i++) {
if (i < (m.size()) / 2)
left.add(m.get(i));
else {
right.add(m.get(i));
}
}
答案2
得分: 0
在mergeSort方法中，需要稍微修改for循环，然后再次尝试。
for (int i = 0; i < m.size() / 2; i++)
left.add(m.get(i));
for (int i = m.size() / 2; i < m.size(); i++)
right.add(m.get(i));
英文:
In the mergeSort method need to change the for loop little bit and try again.
for (int i=0;i< m.size()/2;i++)
left.add(m.get(i));
for (int i=m.size()/2;i< m.size();i++)
right.add(m.get(i));
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