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MergeSort.java
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107 lines (96 loc) · 2.74 KB
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package sorting;
import java.util.Random;
public class MergeSort {
// bottom up merge sort
public void mergeSortBottomUp(int[] arr) {
int L = arr.length;
int[] arrToMerge = new int[L];
for(int i=2; i<L<<1; i=i<<1) {
for(int j=0; j<L; j+=i) {
int left = j;
int right = j+i-1 < L-1 ? j+i-1 : L-1;
int mid = j+(i>>1)-1;
merge(arr, arrToMerge, left, mid, right);
}
}
}
// top down merge sort
public void mergeSortTopDown(int[] arr) {
int[] arrToMerge = new int[arr.length];
mergeSort(arr, arrToMerge, 0, arr.length-1);
}
public void mergeSort(int[] arr, int[] arrToMerge, int left, int right) {
if(left<right) {
int mid = left + ((right-left)>>1);
mergeSort(arr, arrToMerge, left, mid);
mergeSort(arr, arrToMerge, mid+1, right);
merge(arr, arrToMerge, left, mid, right);
}
}
// merge with one 'for' loop,
// reference: http://algs4.cs.princeton.edu/code/edu/princeton/cs/algs4/Merge.java.html
private void merge(int[] arr, int[] arrToMerge, int left, int mid, int right) {
for(int m=left; m<=right; m++)
arrToMerge[m] = arr[m]; // copy arr to arrToMerge
int i=left, j=mid+1;
for(int k=left; k<=right; k++) {
if(i>mid)
arr[k] = arrToMerge[j++];
else if(j>right)
arr[k] = arrToMerge[i++];
else if(arrToMerge[i]<=arrToMerge[j])
arr[k] = arrToMerge[i++];
else
arr[k] = arrToMerge[j++];
}
}
/*
// merge with three 'while' loops
private void merge(int[] arr, int[] arrToMerge, int left, int mid, int right) {
int i=left, j=mid+1;
for(int m=left; m<=right; m++)
arrToMerge[m] = arr[m];
int k = left;
while(i<=mid && j<=right) {
if(arrToMerge[i] <= arrToMerge[j])
arr[k++] = arrToMerge[i++];
else
arr[k++] = arrToMerge[j++];
}
while(i<=mid)
arr[k++] = arrToMerge[i++];
while(j<=right)
arr[k++] = arrToMerge[j++];
}
*/
public static void main(String[] args) {
int N = 10000000;
int[] arr1 = new int[N];
int[] arr2 = new int[N];
Random rm = new Random();
//System.out.print("Original:\t");
for(int i=0; i<N; i++) {
arr1[i] = rm.nextInt(10000);
//System.out.print(arr1[i] + " ");
}
//System.out.println();
arr2 = arr1.clone();
MergeSort ms = new MergeSort();
long start1 = System.nanoTime();
ms.mergeSortTopDown(arr1);
long end1 = System.nanoTime();
//System.out.print("\n\nTD Sorted:\t");
//for(int i : arr1)
// System.out.print(i + " ");
//System.out.println();
long start2 = System.nanoTime();
ms.mergeSortBottomUp(arr2);
long end2 = System.nanoTime();
//System.out.print("\n\nBU Sorted:\t");
//for(int i : arr2)
// System.out.print(i + " ");
//System.out.println();
System.out.println("TD time (ms): " + (end1-start1)/1e6);
System.out.println("BU time (ms): " + (end2-start2)/1e6);
}
}