将两个或两个以上的有序表组合成一个新的有序表。归并排序要使用一个辅助数组，大小跟原数组相同，递归做法。每次将目标序列分解成两个序列，分别排序两个子序列之后，再将两个排序好的子序列merge到一起。
归并排序Java代码：
public class MergeSort {
private double[] bridge;//辅助数组
public void sort(double[] obj){
if (obj == null){
throw new NullPointerException("The param can not be null!");
}
bridge = new double[obj.length]; // 初始化中间数组
mergeSort(obj, 0, obj.length - 1); // 归并排序
bridge = null;
}
private void mergeSort(double[] obj, int left, int right){
if (left < right){
int center = (left + right) / 2;
mergeSort(obj, left, center);
mergeSort(obj, center + 1, right);
merge(obj, left, center, right);
}
}
private void merge(double[] obj, int left,int center, int right){
int mid = center + 1;
int third = left;
int tmp = left;
while (left <= center && mid <= right){
// 从两个数组中取出小的放入中间数组
if (obj[left]-obj[mid]<=10e-6){
bridge[third++] = obj[left++];
} else{
bridge[third++] = obj[mid++];
}
}
// 剩余部分依次置入中间数组
while (mid <= right){
bridge[third++] = obj[mid++];
}
while (left <= center){
bridge[third++] = obj[left++];
}
// 将中间数组的内容拷贝回原数组
copy(obj, tmp, right);
}
private void copy(double[] obj, int left, int right)
{
while (left <= right){
obj[left] = bridge[left];
left++;
}
}
public static void main(String[] args) {
Random random = new Random(6);
int arraysize = 10;
double[] sorted = new double[arraysize];
System.out.print("Before Sort:");
for (int j = 0; j < arraysize; j++) {
sorted[j] = (int) (random.nextDouble() * 100);
System.out.print((int) sorted[j] + " ");
}
System.out.println();
MergeSort sorter = new MergeSort();
sorter.sort(sorted);
System.out.print("After Sort:");
for (int j = 0; j < sorted.length; j++) {
System.out.print((int) sorted[j] + " ");
}
System.out.println();
}
}