归并排序——递归、非递归、小和

1，递归

    public void mergeSort(int[] arr){
        if(arr==null || arr.length<2)   return;
        sort1(arr,0,arr.length-1);
    }

    public void sort1(int[] arr,int l,int r){
        if(l>=r)    return;
        int mid=l+((r-l)>>1);
        sort1(arr,l,mid);
        sort1(arr,mid+1,r);
        merge(arr,l,mid,r);
    }

    public void merge(int[] arr,int l,int mid,int r){
        int[] tmp=new int[r-l+1];
        int i=0;
        int p1=l,p2=mid+1;
        while(p1<=mid&&p2<=r){
            tmp[i++]= arr[p1]<=arr[p2]?arr[p1++]:arr[p2++];
        }
        while(p1<=mid){
            tmp[i++]=arr[p1++];
        }
        while (p2<=r)
            tmp[i++]=arr[p2++];
        i=0;
        while(l<=r){
            arr[l++]=tmp[i++];
        }
    }

2.非递归

    public void mergeSort2(int[] arr){
        if(arr.length<2 || arr==null)   return;
        int n=arr.length;
        int size=1;
        while(size<n){
            int l=0;
            while(l<n){
                int m=l+size-1;
                if(m>=n)    break;
                int r=Math.min(n-1,m+size);
                merge(arr,l,m,r);
                l=r+1;
            }
            if(size>n/2)
                break;
            size<<=1;
        }
    }

3.例题

    @Test
    public void test01(){
        int[] arr={1,3,4,2,5};
        System.out.println(sum(arr));
    }

    public int sum(int[] arr){
        if(arr==null||arr.length<2)
            return 0;
        return sort(arr,0,arr.length-1);
    }

    public int sort(int[] arr,int l,int r){
        if (l>=r)   return 0;
        int mid=l+((r-l)>>1);
        int left=sort(arr,l,mid);
        int right=sort(arr,mid+1,r);
        int merge=merge(arr,l,mid,r);
        return left+right+merge;
    }
    public int merge(int[] arr,int l,int mid,int r){
         int[] tmp=new int[r-l+1];
         int i=0,res=0;
         int p1=l,p2=mid+1;
         while(p1<=mid&&p2<=r){
             res+=arr[p1]<arr[p2]?arr[p1]*(r-p2+1):0;
             tmp[i++]=arr[p1]<=arr[p2]?arr[p1++]:arr[p2++];
         }
         while(p1<=mid){
             tmp[i++]=arr[p1++];
         }
         while(p2<=r)
             tmp[i++]=arr[p2++];
         i=0;
         while(l<r)
             arr[l++]=tmp[i++];
         return res;
    }