This is a Natural Merge Sort program from my textbook. It works, but I don‘t think it‘s good.
// Natural merge sort program in the textbook public class NaturalMergeSortProgram { public static void main(String[] args) { int a[] = new int[10000000]; int b[] = new int[a.length]; for (int i = 0; i < a.length; i++) a[i] = (int)(1+Math.random()*(100-0+1)); long starTime=System.currentTimeMillis(); NaturalMergeSort(a, b); long endTime=System.currentTimeMillis(); long Time = endTime - starTime; System.out.println("executing time: "+Time+"(ms)"); /*for (int i = 0; i < b.length; i++) { if (i % 20 == 0) System.out.println(); System.out.print(a[i]+" "); }*/ } public static void NaturalMergeSort(int a[], int b[]) { // merge array a into b and then b into a until sorted while (!MergeRuns(a, b) & !MergeRuns(b, a)); } public static boolean MergeRuns(int a[], int b[]) { int i = 0, k = 0; int n = a.length; boolean asc = true; int x; while (i < n) { k = i; do x = a[i++]; while (i < n && x <= a[i]); // elements are increasing while (i < n && x >= a[i]) // elements are decreasing x = a[i++]; merge(a, b, k , i-1, asc); asc = !asc; } return k == 0; } public static void merge(int a[], int b[], int low, int high, boolean asc) { // merge a[low:high] into b[low:high] int k = asc ? low : high; int c = asc ? 1 : -1; int i = low, j = high; while (i <= j) { if (a[i] <= a[j]) b[k] = a[i++]; else b[k] = a[j--]; k += c; } } }
Or maybe I don‘t get it? Because it‘s rather obscure and lack of comments( these comments are all added by me).
So I decide to write my own Natural Merge Sort program:
// My own natural merge sort program public class MyNaturalMergeSort { public static void main(String args[]) { int a[] = new int[10000000]; int b[] = new int[a.length]; for (int i = 0; i < a.length; i++) a[i] = (int)(1+Math.random()*(100-0+1)); long starTime=System.currentTimeMillis(); while (!NaturalMergeSort(a, b) & !NaturalMergeSort(b, a)); long endTime=System.currentTimeMillis(); long Time = endTime - starTime; System.out.println("executing time: "+Time+"(ms)"); /*for (int i = 0; i < a.length; i++) System.out.print(a[i]+" "); System.out.println(); */ } public static boolean NaturalMergeSort(int x[], int y[]) { // find the two adjacent natural increasing arrays x[l:m] and x[m+1:r], // then merge them into y[l:r] using function merge() int i, l, m = 0, r; for (i = 0; i < x.length; i++) { l = i; while ((i < x.length-1) && (x[i] <= x[i+1])) // get x[l:m] i++; m = i++; if (m == x.length-1) // the whole array is sorted return true; while ((i < x.length-1) && (x[i] <= x[i+1])) // get x[m+1:r] i++; r = i; merge(x, y, l, m, r); } return false; // the whole array is not yet sorted } public static void merge(int x[], int y[], int l, int m, int r) { // merge x[l:m] and x[m+1:r] into y[l:r] int i = l, j = m+1, k = l; while ((i <= m) && (j <= r)) if (x[i] <= x[j]) y[k++] = x[i++]; else y[k++] = x[j++]; while (k <= r) if (i > m) // elements in x[l:m] are all merged into array y[] y[k++] = x[j++]; else y[k++] = x[i++]; } }
After running each program for 3 times, I got the executing time as below:
program in textbook -- 1205ms 1261ms 1260ms
my program -- 202ms 203ms 198ms
Wow! My program is bad-ass! At least better than the one in textbook. This makes me exciting!
And through this practice, I came to know there‘s a lot of fun hacking the algorithm. I‘m looking forword to write more beautiful and efficient code!
时间: 2024-10-25 12:44:16