Merge sort - four sorted parts in one array - c++

Question

I have one array where are 4 sorted parts. For example
int array[20] = {1,4,7,8,10,2,3,6,8,11,1,2,7,8,9,3,4,9,10,13}
What I need to do is use merge sort on first 2 sorted parts (1,4,7,8,10 and 2,3,6,8,11) and then for second 2 sorted parts (1,2,7,8,9 and 3,4,9,10,13). Then I need to merge these 2 sorted parts into one sorted array.
I tried to use these piece of code, but there is something wrong.
void Merge(int *array, int *aux, int left, int right)
{
int middleIndex = (left + right) / 2;
int leftIndex = left;
int rightIndex = middleIndex + 1;
int auxIndex = left;
while (leftIndex <= middleIndex && rightIndex <= right)
{
if (array[leftIndex] >= array[rightIndex])
{
aux[auxIndex] = array[leftIndex++];
}
else
{
aux[auxIndex] = array[rightIndex++];
}
auxIndex++;
}
while (leftIndex <= middleIndex)
{
aux[auxIndex] = array[leftIndex++];
auxIndex++;
}
while (rightIndex <= right)
{
aux[auxIndex] = array[rightIndex++];
auxIndex++;
}
}
Any idea how to modify this, or write it better? Thanks

Answer 1

import java.util.Arrays;
public class Main {
public static void main(String[] args) {
int[] sortArr1 = merge(new int[] {1 , 4 , 7 , 8 , 10} , new int[] {2 , 3 , 6 , 8 , 11} );
System.out.println("Merging of sorted array " +Arrays.toString(sortArr1));
int[] sortArr2 = merge(new int[] {1 , 2 , 7 , 8 , 9} , new int[] {3 , 4 , 9 , 10 , 13} );
System.out.println("Merging of sorted array " +Arrays.toString(sortArr2));
int[] finalSort = merge(sortArr1 , sortArr2);
System.out.println("Merging of sorted array " +Arrays.toString(finalSort));
}
public static int[] merge(int[] arr1 , int[] arr2) {
int[] sort = new int[arr1.length + arr2.length];
int j = 0;
int k = 0;
for(int i = 0 ; i < sort.length ; i++ ) {
if(j <= (arr1.length - 1) && k <= (arr2.length - 1)) {
if(arr1[j] > arr2[k]) {
sort[i] = arr2[k++];
}else {
sort[i] = arr1[j++];
}
}else if(j <= (arr1.length - 1)) {
sort[i] = arr1[j++];
}else if(k <= (arr2.length - 1)){
sort[i] = arr2[k++];
}
}
return sort;
}
}
Output of the program is :
Merging of sorted array [1, 2, 3, 4, 6, 7, 8, 8, 10, 11]
Merging of sorted array [1, 2, 3, 4, 7, 8, 9, 9, 10, 13]
Merging of sorted array [1, 1, 2, 2, 3, 3, 4, 4, 6, 7, 7, 8, 8, 8, 9, 9, 10, 10, 11, 13]

Answer 2

Any idea how to modify this, or write it better? Thanks
Since the ranges are already sorted, and you know where the ranges start and end, use std::inplace_merge:
#include <algorithm>
#include <iostream>
#include <iterator>
int array[20] = {1,4,7,8,10,2,3,6,8,11,1,2,7,8,9,3,4,9,10,13};
using namespace std;
void MergeSort(int *arr, int start1, int start2, int start3,
int start4, int size)
{
std::inplace_merge(arr + start1, arr + start2, arr + start3);
std::inplace_merge(arr + start3, arr + start4, arr + size);
std::inplace_merge(array, array + start3, arr + size);
}
int main()
{
MergeSort(array, 0, 5, 10, 15, 20);
std::copy(array, array + 20, ostream_iterator<int>(cout, " "));
}
Live Example