4.归并排序（稳定）

public static void main(String[] args) {
        int[] arr = {49, 38, 65, 97, 76, 13, 27, 50};
        //int[] c = { 13, 38, 65, 97, 76, 13, 2, 50 };//稳定性判断
        rec(arr,0,arr.length-1);
        for (int i : arr) {
            System.out.print(i+" ");
        }
    }
    public static void rec(int[] arr,int l,int r) {
        if (l==r)return;
        int mid=l+((r-l)>>1);
        rec(arr,l,mid);
        rec(arr,mid+1,r);
        merge(arr,l,mid,r);
    }
    public static void merge(int[] arr,int l,int mid,int r) {
        int i=0;
        int p=l;
        int q=mid+1;
        int[] help=new int[r-l+1];
        while (p<=mid&&q<=r){
            help[i++]=arr[p]<arr[q]?arr[p++]:arr[q++];
        }
        while (p<=mid)
        {
            help[i++]=arr[p++];
        }
        while (q<=r)
        {
            help[i++]=arr[q++];
        }
        for (int i1 = 0; i1 < help.length; i1++) {
            arr[l+i1]=help[i1];
        }
    }