核心code：

package com.sort;

public class mergesort {
    public static void main(String[] args) {
        int[] a = {1, 4, 3, 5, 2, 7, 12, 45, 13, 754, 124, 754, 6, 32, 45, 1};
        mergesort mergesort = new mergesort();
        mergesort.merge(a);
        for (int b : a) System.out.println(b);
    }

    public void merge(int[] a){
        merge1(a,0,a.length-1);
    }

    private void merge1(int[] a,int l,int r){
        int mid = (l+r)/2;
        merge1(a,0,mid);
        merge1(a,mid+1,r);
        sort(a,l,mid+1,r);
    }

    private void sort(int[] a,int l,int m,int r){
        int[] q = new int[r-l+1];
        int start = l ;
        int mid=m;
        int t=0;
        while(l<mid||m<=r){
            if(a[l]<=a[m]) q[t++]=a[l++];
            else q[t++]=a[m++];
        }
        if(l==mid) while (m<=r) q[t++]=a[m++];
        else while (l<mid) q[t++]=a[l++];
        for(int i=0;i<q.length;i++) a[start++]=q[i];
    }
}