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144 lines (126 loc) · 2.85 KB
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package algorithms;
import java.util.Arrays;
import java.util.Random;
/**
* Merge sort algorithm (Top down) based on pseudocode at Wikipedia "Merge sort" article.
*
* @see <a href="http://en.wikipedia.org/wiki/Merge_sort"/>
* @author djitz
*/
public class MergeSort
{
public static void main(String[] args)
{
//Generate an integer array of length 7
int[] input = generateRandomNumbers(7);
//Before sort
System.out.println(Arrays.toString(input));
//After sort
System.out.println(Arrays.toString(mergeSort(input)));
}
/**
* Sorts the input array using top-down merge sort algorithm.
* @param the array of integers to sort
* @return sorted array of integers
*/
private static int[] mergeSort(int[] input)
{
if(input.length == 1)
{
return input;
}
int middle = (int) Math.ceil((double)input.length / 2);
int[] left = new int[middle];
int rightLength = 0;
if(input.length % 2 == 0)
{
rightLength = middle;
}
else
{
rightLength = middle - 1;
}
int[] right = new int[rightLength];
int leftIndex = 0;
int rightIndex = 0;
for (int i = 0; i < input.length; i++)
{
if(i < middle)
{
left[leftIndex] = input[i];
leftIndex++;
}
else
{
right[rightIndex] = input[i];
rightIndex++;
}
}
left = mergeSort(left);
right = mergeSort(right);
return merge(left, right);
}
/**
* Merges two integer arrays into a sorted integer array.
* @param left first array
* @param right second array
* @return a sorted integer array
*/
private static int[] merge(int[] left, int[] right)
{
int[] result = new int[left.length + right.length];
int leftIndex = 0;
int rightIndex = 0;
int resultIndex = 0;
while(leftIndex < left.length || rightIndex < right.length)
{
if(leftIndex < left.length && rightIndex < right.length)
{
if(left[leftIndex] < right[rightIndex])
{
result[resultIndex] = left[leftIndex];
leftIndex++;
resultIndex++;
}
else
{
result[resultIndex] = right[rightIndex];
rightIndex++;
resultIndex++;
}
}
else if(leftIndex < left.length)
{
for (int i = resultIndex; i < result.length; i++)
{
result[i] = left[leftIndex];
leftIndex++;
}
}
else if(rightIndex < right.length)
{
for (int i = resultIndex; i < result.length; i++)
{
result[i] = right[rightIndex];
rightIndex++;
}
}
}
return result;
}
/**
* Generates an array of random integers with length n.
* @param the length of the array to generate
* @return array of random integers with length n
*/
private static int[] generateRandomNumbers(int n)
{
int[] result = new int[n];
Random random = new Random();
for (int i = 0; i < result.length; i++)
{
result[i] = random.nextInt(n * 10);
}
return result;
}
}