how does merge sort work java code example

Example 1: merge sort java

public static void mergeSort(int[] a, int n) {
    if (n < 2) {
        return;
    }
    int mid = n / 2;
    int[] l = new int[mid];
    int[] r = new int[n - mid];
 
    for (int i = 0; i < mid; i++) {
        l[i] = a[i];
    }
    for (int i = mid; i < n; i++) {
        r[i - mid] = a[i];
    }
    mergeSort(l, mid);
    mergeSort(r, n - mid);
 
    merge(a, l, r, mid, n - mid);
}

public static void merge(
  int[] a, int[] l, int[] r, int left, int right) {
 
    int i = 0, j = 0, k = 0;
    while (i < left && j < right) {
        if (l[i] <= r[j]) {
            a[k++] = l[i++];
        }
        else {
            a[k++] = r[j++];
        }
    }
    while (i < left) {
        a[k++] = l[i++];
    }
    while (j < right) {
        a[k++] = r[j++];
    }
}

Example 2: Merge sort java

public class JavaMergeSort
{
   void sorting(int[] num, int left, int main, int right)
   {
      // finding size of two sub arrays
      int list1 = main - left + 1;
      int list2 = right - main;
      // creating temporary array
      int[] L = new int[list1];
      int[] R = new int[list2];
      // copying data to temporary array
      for(int a = 0; a < list1; ++a)
         L[a] = num[left + a];
      for(int b = 0; b < list2; ++b)
         R[b] = num[main + 1+ b];
      // existing index of first and second sub array
      int p = 0, q = 0;
      // existing index of merged sub array
      int r = left;
      while(p < list1 && q < list2)
      {
         if(L[p] <= R[q])
         {
            num[r] = L[p];
            p++;
         }
         else
         {
            num[r] = R[q];
            q++;
         }
         r++;
      }
      // copying remaining elements of L[] array
      while(p < list1)
      {
         num[r] = L[p];
         p++;
         r++;
      }
      // copying remaining elements of R[] array
      while(q < list2)
      {
         num[r] = R[q];
         q++;
         r++;
      }
   }
   // function that sorts
   void sort(int[] arrNum, int l, int r)
   {
      if(l < r)
      {
         // finding middle point
         int m = (l + r) / 2;
         // sorting first and second list
         sort(arrNum, l, m);
         sort(arrNum , m+1, r);
         // merging sorted list
         sorting(arrNum, l, m, r);
      }
   }
   // display array
   static void displayArray(int[] arr)
   {
      int number = arr.length;
      for(int a = 0; a < number; ++a)
         System.out.print(arr[a] + " ");
      System.out.println();
   }
   public static void main(String[] args)
   {
      int[] arrNumbers = {33, 00, 55, 11, 22, 44};
      System.out.println("Before sorting - ");
      displayArray(arrNumbers);
      JavaMergeSort obj = new JavaMergeSort();
      obj.sort(arrNumbers, 0, arrNumbers.length - 1);
      System.out.println("\nAfter sorting - ");
      displayArray(arrNumbers);
   }
}

Example 3: mergesort java

public class MergeSort
{
public static void main(String[] args)
{
int i;
int values[] = new int[11];
for(i=0; i<values.length; i++) { values[i]=values.length-i; }
print(values);
sort(values,0,values.length-1);
print(values);
}
public static void sort(int[] numbers, int p,int r)
{
int q;
if(p<r)
{
q = (p+r)/2;
sort(numbers,p,q);
sort(numbers,q+1,r);
merge(numbers,p,q,r);
}
}
/**
* p, mid, and r are indices into the array such that p <= mid < r.
* The procedure assumes that the subarrays 'numbers[p..mid]' and 'numbers[mid+1..r]' are
* in sorted order. It merge them to form a single sorted subarray that replaces
* the current subarray 'numbers[p..r]'.
*/
private static void merge(int[] values, int p, int mid, int r)
{
int i,j,k;
int n1 = mid - p + 1;
int n2 = r - mid;
int[] left = new int[n1+1];
int[] right = new int[n2+1];
for(i=0; i<n1; i++)
{
left[i] = values[ p + i ];
}
for(j=0; j<n2; j++)
{
right[j] = values[mid + j + 1];
}
left[n1] = Integer.MAX_VALUE;
right[n2] = Integer.MAX_VALUE;
i=0; j=0;
for(k=p; k<=r; k++)
{
if(left[i]<=right[j])
{
values[k] = left[i];
i = i + 1;
}
else
{
values[k] = right[j];
j = j + 1;
}
}
}
public static void print(int[] numbers)
{
int i;
for(i=0; i<numbers.length; i++)
{
System.out.print(numbers[i]+ " ");
}
System.out.println();
}
}