// 493. Reverse Pairs #include "leetcode.h" /* * given an integer array 'nums', return the number of reverse pairs in the * array. a reverse pair is a pair '(i, j)' where '0 <= i < j < nums.len' and * 'nums[i] > 2 * nums[j]' */ int merge_cnt(int *arr, int *tmp, int left, int mid, int right) { int i = left, j = mid + 1, k = left, cnt = 0; while (i <= mid && j <= right) { if ((long long)arr[i] > 2 * (long long)arr[j]) { cnt += (mid - i + 1); j++; } else i++; } i = left; j = mid + 1; while (i <= mid && j <= right) tmp[k++] = arr[i] <= arr[j] ? arr[i++] : arr[j++]; while (i <= mid) tmp[k++] = arr[i++]; while (j <= right) tmp[k++] = arr[j++]; for (int l = left; l <= right; l++) arr[l] = tmp[l]; return cnt; } int sort_merge_cnt(int *arr, int *tmp, int left, int right) { int cnt = 0; if (left < right) { int mid = left + (right - left) / 2; cnt += sort_merge_cnt(arr, tmp, left, mid); cnt += sort_merge_cnt(arr, tmp, mid + 1, right); cnt += merge_cnt(arr, tmp, left, mid, right); } return cnt; } int reversePairs(int *nums, int numsSize) { int *tmp = (int *)malloc(numsSize * sizeof(int)); return sort_merge_cnt(nums, tmp, 0, numsSize - 1); } int main() { int n1[] = {1, 3, 2, 3, 1}, n2[] = {2, 4, 3, 5, 1}; printf("%d\n", reversePairs(n1, ARRAY_SIZE(n1))); // expect: 2 printf("%d\n", reversePairs(n2, ARRAY_SIZE(n2))); // expect: 3 }