在一个数组中,左边的数如果比右边的数大,则这两个数构成一个逆序对。
暴力解法
public int comparator(int[] arr) {
int ans = 0;
for (int i = 0; i < arr.length; i++) {
for (int j = i + 1; j < arr.length; j++) {
if (arr[i] > arr[j])
ans++;
}
}
return ans;
}
归并排序批量计算
public int ReverPairNumber(int[] arr) {
if (arr == null || arr.length < 0)
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 = help.length - 1, p1 = mid, p2 = r, res = 0;
while (p1 >= l && p2 >= mid) {
res += arr[p1] > arr[p2] ? (p2 - mid) : 0;
help[i--] = arr[p1] > arr[p2] ? arr[p1--] : arr[p2--];
}
while (p1 >= l)
help[i--] = arr[p1--];
while (p2 > mid)
help[i--] = arr[p2--];
for (i = 0; i < help.length; i++)
arr[l + i] = help[i];
return res;
}