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Wednesday, February 26, 2014

Merge sort in java

Splitting the array into sets and compare / merge them during iterations. The time complexity is O(n log n) 
import java.util.*;

public class MergeSort {
public static void main(String[] args) {
int[] list = {14, 32, 67, 76, 23, 41, 58, 85};
System.out.println("before: " + Arrays.toString(list));
mergeSort(list);
System.out.println("after: " + Arrays.toString(list));
}

// Places the elements of the given array into sorted order
// using the merge sort algorithm.
// post: array is in sorted (nondecreasing) order
public static void mergeSort(int[] array) {
if (array.length > 1) {
// split array into two halves
int[] left = leftHalf(array);
int[] right = rightHalf(array);

// recursively sort the two halves
mergeSort(left);
mergeSort(right);

// merge the sorted halves into a sorted whole
merge(array, left, right);
}
}

// Returns the first half of the given array.
public static int[] leftHalf(int[] array) {
int size1 = array.length / 2;
int[] left = new int[size1];
for (int i = 0; i < size1; i++) {
left[i] = array[i];
}
return left;
}

// Returns the second half of the given array.
public static int[] rightHalf(int[] array) {
int size1 = array.length / 2;
int size2 = array.length - size1;
int[] right = new int[size2];
for (int i = 0; i < size2; i++) {
right[i] = array[i + size1];
}
return right;
}

// Merges the given left and right arrays into the given
// result array. Second, working version.
// pre : result is empty; left/right are sorted
// post: result contains result of merging sorted lists;
public static void merge(int[] result,
int[] left, int[] right) {
int i1 = 0; // index into left array
int i2 = 0; // index into right array

for (int i = 0; i < result.length; i++) {
if (i2 >= right.length || (i1 < left.length &&
left[i1] <= right[i2])) {
result[i] = left[i1]; // take from left
i1++;
} else {
result[i] = right[i2]; // take from right
i2++;
}
}
}
}

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