归并排序

算法效率(O(N*LogN))优于冒泡，选择以及插入排序（O(N*N)），这里的实现效率不高，生成了大量临时数组，对于理解比较容易。

 

public class Main {

    public static void main(String[] args) {
        int [] a = {9,8,7,6,5,4,3,2,1};
        int [] b = mergeSort(a) ;
        for (int i:b)
            System.out.println(i);
    }
   
    //将有序数组a和b合并到数组c中
    private static int [] merge(int [] a, int [] b){
        int [] c = new int[a.length + b.length] ;
        int i=0,j=0,m=0 ;
        while(i<a.length && j<b.length)
        {
            if (a[i] < b[j])
               c[m++] = a[i++];
            else
                c[m++] = b[j++];
        }
        if (i < a.length)
            while(i<a.length)
              c[m++] = a[i++];

        if (j < b.length)
            while(j < b.length)
                c[m++] = b[j++];
        return c ;
    }

    private static int [] mergeSort(int [] a){
        if ( a.length == 1)
            return a ;
        else
        {
            int [] p1 = mergeSort(Arrays.copyOfRange(a, 0, a.length/2 ));
            int [] p2 = mergeSort(Arrays.copyOfRange(a,  a.length/2, a.length ));
            return merge(p1,p2);
        }
    }
}