求小和---由归并排序改写得到

public class 求小和 {
    public static void main(String[] args) {
        int arr[] = {1,3,4,2,5};
        System.out.println(smallSum(arr));
        for(int a:arr){
            System.out.print(a+" ");
        }
    }

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

    public static int process(int []arr, int L, int R){
        if(L == R)return 0;
        int mid = L + ((R - L) >> 1);
        return process(arr,L,mid)+
                process(arr,mid+1,R)+
                merge(arr,L,mid,R);
    }

    public static int 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;
        int sum = 0;
        while(p1 <= M && p2 <= R){
            sum += arr[p1] < arr[p2] ? (R - p2 + 1) * arr[p1]: 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(i = 0 ; i < help.length; i++){
            arr[L++] = help[i];
        }
        return sum;
    }
}

代码来自视频学习：https://www.bilibili.com/video/BV13g41157hK?p=3