3.归并排序

public static void merge(int[]arr,int L,int M,int R){
     int[]help=new int[R-L+1];
     int i=0;
     int p1=L;
     int p2=M+1;
     while(p1<=M && p2<=R){
          help[i++]=arr[p1]<=arr[p2]?arr[p1++]:arr[p2++];
     }
     while(p1<=M){
          help[i++]=arr[p1++];
     }
     while(p2<=R){
          help[i++]=arr[p2++];
     }
     for(i=0;i<help.length;i++){
          arr[L+i]=help[i];
     }
}