diff options
Diffstat (limited to 'reverse_pairs.c')
-rw-r--r-- | reverse_pairs.c | 52 |
1 files changed, 52 insertions, 0 deletions
diff --git a/reverse_pairs.c b/reverse_pairs.c new file mode 100644 index 0000000..261a9cf --- /dev/null +++ b/reverse_pairs.c @@ -0,0 +1,52 @@ +// 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 +} |