自顶向下的归并排序采用了分治的思想：将两个已排序的较小的序列“归并”为一个较大的数组。简易实现以用例如下：
1 public class Merge { 2 3 private static Comparable[] aux; 4 private static void exch(Comparable[] a, int i, int j) { 5 Comparable tmp = a[i]; 6 a[i] = a[j]; 7 a[j] = tmp; 8 } 9 10 private static boolean less(Comparable a, Comparable b) { 11 return (a.compareTo(b) < 0); 12 } 13 14 private static void show(Comparable[] a) { 15 int N = a.length; 16 for (int i = 0; i < N; i++) { 17 System.out.print(a[i] + " "); 18 } 19 System.out.println(); 20 } 21 22 public static boolean isSorted(Comparable[] a) { 23 int N = a.length; 24 for (int i = 1; i < N; i++) { 25 if (less(a[i], a[i - 1])) 26 return false; 27 } 28 return true; 29 } 30 31 private static void merge(Comparable[] a, int lo, int mid, int hi) 32 { 33 for(int k = lo; k <= hi; k++) 34 { 35 aux[k] = a[k]; 36 } 37 int i = lo; 38 int j = mid + 1; 39 for(int k = lo; k <= hi; k++) 40 { 41 if( i > mid) a[k] = aux[j++]; 42 else if(j > hi) a[k] = aux[i++]; 43 else if(less(aux[j], aux[i])) a[k] = aux[j++]; 44 else a[k] = aux[i++]; 45 } 46 } 47 48 private static void sort(Comparable[] a, int lo, int hi) 49 { 50 if(hi <= lo) return; 51 int mid = lo + (hi - lo) / 2; 52 sort(a, lo, mid); 53 sort(a, mid + 1, hi); 54 merge(a, lo, mid, hi); 55 } 56 57 public static void sort(Comparable[] a) 58 { 59 aux =new Comparable[a.length]; 60 sort(a, 0, a.length - 1); 61 } 62 public static void main(String[] args) { 63 String[] a = In.readStrings(); 64 sort(a); 65 show(a); 66 } 67 68 }
与此相对应的自底向上的归并排序为：
1 import java.awt.datatransfer.StringSelection; 2 3 4 public class MergeBP { 5 6 private static Comparable[] aux; 7 private static void exch(Comparable[] a, int i, int j) { 8 Comparable tmp = a[i]; 9 a[i] = a[j]; 10 a[j] = tmp; 11 } 12 13 private static boolean less(Comparable a, Comparable b) { 14 return (a.compareTo(b) < 0); 15 } 16 17 private static void show(Comparable[] a) { 18 int N = a.length; 19 for (int i = 0; i < N; i++) { 20 System.out.print(a[i] + " "); 21 } 22 System.out.println(); 23 } 24 25 public static boolean isSorted(Comparable[] a) { 26 int N = a.length; 27 for (int i = 1; i < N; i++) { 28 if (less(a[i], a[i - 1])) 29 return false; 30 } 31 return true; 32 } 33 34 private static void merge(Comparable[] a, int lo, int mid, int hi) 35 { 36 for(int k = lo; k <= hi; k++) 37 { 38 aux[k] = a[k]; 39 } 40 int i = lo; 41 int j = mid + 1; 42 for(int k = lo; k <= hi; k++) 43 { 44 if( i > mid) a[k] = aux[j++]; 45 else if(j > hi) a[k] = aux[i++]; 46 else if(less(aux[j], aux[i])) a[k] = aux[j++]; 47 else a[k] = aux[i++]; 48 } 49 } 50 51 public static void sort(Comparable[] a) 52 { 53 int N = a.length; 54 aux = new Comparable[N]; 55 for(int sz = 1; sz < N; sz += sz) 56 { 57 for(int lo = 0; lo < N - sz; lo += sz + sz) 58 { 59 merge(a, lo, lo + sz - 1, Math.min(lo + sz + sz - 1, N - 1)); 60 } 61 } 62 } 63 public static void main(String[] args) { 64 String[] a = In.readStrings(); 65 sort(a); 66 show(a); 67 } 68 69 }