归并排序 和 小和问题

package com.aixuexi.contact;

public class Mergesort {
    
    public static void mergeSort(int arr[]) {
        if(arr == null || arr.length <= 2) {
            return ;
        }
        mergeSort(arr,0,arr.length-1);
    }
    public static void mergeSort(int arr[],int L,int R) {
        
        if(L == R) {
            return ;
        }
        int mid = (L + R ) / 2;
        mergeSort(arr,L,mid);
        mergeSort(arr,mid+1,R);
        merge(arr,L, mid, R);
    }
    public static void merge(int arr[],int L,int mid,int R) {
        int arr1[] = new int[R -  L + 1];
        int i = 0;
        int p1 = L;
        int p2 = mid + 1;
        while(p1 <= mid && p2 <= R) {
            arr1[i++] = arr[p1] < arr[p2] ? arr[p1++] : arr[p2++];
        }
        
        while (p1 <= mid) {
            arr1[i++] = arr[p1++];
        }
        while (p2 <= R) {
            arr1[i++] = arr[p2++];
        }
        for (i = 0; i < arr1.length; i++) {
            arr[L + i] = arr1[i];
        }
        
    }
    
    public static void main(String[] args) {
        int arr[] = new int[] {1,4,8,3,7,3,43,43,6,8,87,9,4,67};
        mergeSort(arr);
        for(int i = 0; i < arr.length; i++) {
            System.out.println(arr[i]);
        }
    }
}

package com.aixuexi.contact;

/* 2019/7/16  09点25分
 * 
 * 总结：     1个问题行数   --28 行
 * 注意点     38行
 * 
 * return mergeSort(arr, l, mid) + mergeSort(arr, mid + 1, r) + merge(arr, l, mid, r);
 * res += arr[p1] < arr[p2] ? (r - p2 + 1) * arr[p1] : 0; //算下标
 */

public class SmallSum {

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

    public static int mergeSort(int[] arr, int l, int r) {
        if (l == r) {
            return 0;
        }
        int mid = l + ((r - l) >> 1); 
//        mergeSort(arr, l, mid); // 0
//        mergeSort(arr, mid + 1, r); // 0
//        int ret = merge(arr, l, mid, r); //ret = 9
        
        return mergeSort(arr, l, mid) + mergeSort(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 res = 0;
        while (p1 <= m && p2 <= r) {
            res += arr[p1] < arr[p2] ? (r - p2 + 1) * arr[p1] : 0; //算下标
            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];
        }
        return res;
    }
    
    public static void main(String[] args) {
        int arr[] = new int[] {1,3,4,2,5};
        int res = smallSum(arr);
        System.out.println(res);
//        for(int i = 0; i < arr.length; i++) {
//            System.out.println(arr[i]);
//        }
        
    }

}