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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
}
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