Algorithm-Implementations/MergeSort.java at master · devForTheFuture/Algorithm-Implementations · GitHub

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packagealgorithms;

importjava.util.Arrays;
importjava.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
 */
publicclassMergeSort
{
publicstaticvoidmain(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
 */
privatestaticint[] mergeSort(int[] input)
	{
if(input.length == 1)
 {
returninput;
 }

intmiddle = (int) Math.ceil((double)input.length / 2);
int[] left = newint[middle];

intrightLength = 0;
if(input.length % 2 == 0)
 {
rightLength = middle;
 }
else
 {
rightLength = middle - 1;
 }
int[] right = newint[rightLength];

intleftIndex = 0;
intrightIndex = 0;

for (inti = 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);

returnmerge(left, right);
	}

/**
 * Merges two integer arrays into a sorted integer array.
 * @param left first array
 * @param right second array
 * @return a sorted integer array
 */
privatestaticint[] merge(int[] left, int[] right)
	{
int[] result = newint[left.length + right.length];
intleftIndex = 0;
intrightIndex = 0;
intresultIndex = 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++;
 }
 }
elseif(leftIndex < left.length)
 {
for (inti = resultIndex; i < result.length; i++)
 {
result[i] = left[leftIndex];
leftIndex++;
 }
 }
elseif(rightIndex < right.length)
 {
for (inti = resultIndex; i < result.length; i++)
 {
result[i] = right[rightIndex];
rightIndex++;
 }
 }
 }

returnresult;
	}

/**
 * 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
 */
privatestaticint[] generateRandomNumbers(intn)
	{
int[] result = newint[n];
Randomrandom = newRandom();

for (inti = 0; i < result.length; i++)
 {
result[i] = random.nextInt(n * 10);
 }

returnresult;
	}
}