在一个数组中,每一个数左边比当前数小的数累加起来,叫做这个数组的小和。求一个数组的小和。
暴力解法
public int comparator(int[] arr) {
if (arr == null || arr.length < 2)
return 0;
int res = 0;
for (int i = 1; i < arr.length; i++) {
for (int j = 0; j < i; j++) {
res += arr[j] < arr[i] ? arr[j] : 0;
}
}
return res;
}
归并排序批量计算
public int smallSum(int[] arr) {
if (arr == null || arr.length < 2)
return 0;
return process(arr, 0, arr.length - 1);
}
private 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);
}
private int merge(int[] arr, int l, int mid, int r) {
int[] help = new int[r - l + 1];
int i = 0, p1 = l, p2 = mid + 1, res = 0;
while (p1 <= mid && p2 <= r) {
res += arr[p1] < arr[p2] ? arr[p1] * (r - p2 + 1) : 0;
help[i++] = arr[p1] < arr[p2] ? arr[p1++] : arr[p2++];
}
while (p1 <= mid)
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;
}