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package Algorithms.sort;

/********************************************************

*

* 08-722 Data Structures for Application Programmers

* Lecture 14 Advanced Sorting

*

* Naive version of Merge Sort

*

*********************************************************/

import java.util.Arrays;

public class MergeSort {

private static final int SIZE = 10000;

public static int[] mergeSort(int[] data) {

// parameter valid judge.

if (data == null) {

return null;

}

// the base case.

int len = data.length;

if (len <= 1) {

return data;

}

// divide into two arrays.

// create left half and right half.

int left[] = new int[len/2];

int right[] = new int[len - len/2];

System.arraycopy(data, 0, left, 0, len/2);

System.arraycopy(data, len/2, right, 0, len - len/2);

// call itself to sort left half

left = mergeSort(left);

right = mergeSort(right);

return merge(left, right);

}

// Precondition: two input arrays are sorted and they are not null.

private static int[] merge(int[] left, int[] right) {

int len = left.length + right.length;

int[] ret = new int[len];

int leftPoint = 0;

int rightPoint = 0;

int cur = 0;

while (leftPoint < left.length && rightPoint < right.length) {

if (left[leftPoint] < right[rightPoint]) {

ret[cur++] = left[leftPoint++];

} else {

ret[cur++] = right[rightPoint++];

}

}

if (leftPoint >= left.length) {

while (rightPoint < right.length) {

ret[cur++] = right[rightPoint++];

}

} else {

while (leftPoint < left.length) {

ret[cur++] = left[leftPoint++];

}

}

return ret;

}

public static void main(String[] args) {

int[] a = new int[SIZE];

for (int i = 0; i < SIZE; i++)

a[i] = (int) (Math.random() * SIZE);

//mergeSort(a);

int[] test = { 42, 12, 89, 27, 94, 63, 3, 78 };

System.out.println(Arrays.toString(mergeSort(test)));

// test merge method

int[] left = { 12, 42, 63, 89 };

int[] right = { 3, 27, 78, 94 };

System.out.println(Arrays.toString(merge(left, right)));

// test merge method

int[] left2 = {};

int[] right2 = { 3, 27, 78, 94 };

System.out.println(Arrays.toString(merge(left2, right2)));

}

}