小和问题

 

 

public class SmallSum {
    public static void mergeSort(int[] arr) {
        if (arr == null || arr.length < 2) {
            return;
        }
        mergeSort(arr, 0, arr.length - 1);
    }
    static int sum=0;
    static void mergeSort(int a[],int l,int r) {
        if(l==r) {
            return;
        }
        int m = l+((r-l)>>1);
        mergeSort(a, l, m);
        mergeSort(a, m+1, r);
        merge(a,l,m,r);
    }
    
    static void merge(int a[],int l,int m,int r) {
        int[] help = new int[r-l+1];
        int p1=l;
        int p2=m+1;
        int i=0;
        while(p1<=m&&p2<=r) {
            sum+=a[p1]<a[p2]?a[p1]*(r-p2+1):0;
            help[i++] = a[p1]<a[p2]?a[p1++]:a[p2++];
        }
        
        while(p1<=m) {
            help[i++]=a[p1++];
        }
        
        while(p2<=r) {
            help[i++]=a[p2++];
        }
        
        for(i=0;i<r-l+1;i++) {
            a[l+i]=help[i];
        }
    }
    
    public static void main(String[] args) {
        int a[] = {1,3,4,2,5};
        mergeSort(a);
        System.out.println(sum);
    }
}