Merge Sort
Merge Sort Hacks
Hack 1/2 (works with any comparable type)
class MergeSort<T extends Comparable<T>> {
public void sort(List<T> arr, int l, int r)
{
if (l < r) {
// find midpoint
int m = l + (r - l) / 2;
// sort top and bottom half
sort(arr, l, m);
sort(arr, m + 1, r);
// merge the two sub arrays
merge(arr, l, m, r);
}
}
private void merge(List<T> arr, int l, int m, int r)
{
// Find the sizes of two subarrays to be merged
int n1 = m - l + 1;
int n2 = r - m;
/* Create temp arrays */
List<T> L = new ArrayList<T>(n1);
List<T> R = new ArrayList<T>(n2);
/* Copy data to temp arrays */
for (int i = 0; i < n1; ++i)
L.set(i, arr.get(l + i));
for (int j = 0; j < n2; ++j)
R.set(j, arr.get(m + 1 + j));
/* Merge the temp arrays */
// Initial indexes of first and second subarrays
int i = 0, j = 0;
// Initial index of merged subarray array
int k = l;
while (i < n1 && j < n2) {
if (L.get(i).compareTo(R.get(j)) <= 0) {
arr.set(k, L.get(i));
i++;
}
else {
arr.set(k, R.get(i));
j++;
}
k++;
}
/* Copy remaining elements of L[] if any */
while (i < n1) {
arr.set(k, L.get(i));
i++;
k++;
}
/* Copy remaining elements of R[] if any */
while (j < n2) {
arr.set(k, R.get(j));
j++;
k++;
}
}
}