Java归并法求逆序对(为什么help数组越界了)

public static void main(String[] args) {
        int[]arr=new int[]{4,3,2,1,5,7,6,5,2};
        System.out.println(process(arr,0,arr.length-1));
    }

    public  static int process(int[]arr,int L,int r){
        if(L==r)return 0;
        int M=L+(r-L)/2;
        return
        process(arr,L,M)+
        process(arr,M+1,r)+
        merge(arr,L,M,r);
    }


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