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-rw-r--r--reverse_pairs.c52
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+// 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
+}