leetcode-java/src/sort/MergeSort.java at master · fluency03/leetcode-java · GitHub

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/**
 * MergeSort: It divides input array in two halves, calls itself for the two
 * halves and then merges the two sorted halves.
 *
 * Time Complexity: O(nLogn) in all 3 cases (worst, average and best).
 * Space Complexity: O(n).
 *
 */

importjava.util.Arrays;

publicclassMergeSort {
publicstaticvoidsort(int[] arr) {
mergeSort(arr, 0, arr.length-1);
// int[] helper = new int[arr.length];
// mergeSort(arr, helper, 0, arr.length-1);
 }

privatestaticvoidmergeSort(int[] arr, intl, intr) {
if (l >= r) return;

intm = (l+r)/2;
mergeSort(arr, l, m);
mergeSort(arr, m+1, r);

merge(arr, l, m, r);
 }

privatestaticvoidmerge(int[] arr, intl, intm, intr) {
intlen1 = m - l + 1;
intlen2 = r - m;

intL[] = newint [len1];
intR[] = newint [len2];

for (inti=0; i < len1; i++)
L[i] = arr[l + i];
for (intj=0; j < len2; j++)
R[j] = arr[m + 1 + j];

inti = 0;
intj = 0;
intk = l;
while (i < len1 && j < len2) {
if (L[i] <= R[j]) {
arr[k] = L[i];
i++;
 } else {
arr[k] = R[j];
j++;
 }
k++;
 }

while (i < len1) {
arr[k] = L[i];
i++;
k++;
 }

while (j < len2) {
arr[k] = R[j];
j++;
k++;
 }
 }

//
privatestaticvoidmergeSort(int[] arr, int[] helper, intl, intr) {
if (l >= r) return;

intm = (l+r)/2;
mergeSort(arr, helper, l, m);
mergeSort(arr, helper, m+1, r);

merge(arr, helper, l, m, r);
 }

privatestaticvoidmerge(int[] arr, int[] helper, intl, intm, intr) {
for (inti = l; i <= r; i++) {
helper[i] = arr[i];
 }

inti = l;
intj = m + 1;
intk = l;
while (i <= m && j <= r) {
if (helper[i] <= helper[j]) {
arr[k] = helper[i];
i++;
 } else {
arr[k] = helper[j];
j++;
 }
k++;
 }

while (i <= m) {
arr[k] = helper[i];
k++;
i++;
 }
 }

publicstaticvoidmain(String[] args) {
int[] arr1 = {10, 3, 7, 5, 20, 15, 1};
MergeSort.sort(arr1);
System.out.println(Arrays.toString(arr1));

int[] arr2 = {};
MergeSort.sort(arr2);
System.out.println(Arrays.toString(arr2));

int[] arr3 = {10};
MergeSort.sort(arr3);
System.out.println(Arrays.toString(arr3));
 }

}