// 2563. Count the Number of Fair Pairs #include "leetcode.h" /* * given a 0 indexed integer array 'nums' of size 'n', and two integers 'lower' * and 'upper', return the number of fair pairs. pair '(i, j)' is fair is '0 <= * i < j < n' and 'lower <= nums[i] + nums[j] <= upper' */ int cmp(const void *a, const void *b) { return *(int *)a - *(int *)b; } int bs_lower(int *nums, int n, int left, int right, int lower) { int l = left, r = right; if (nums[r] < lower) return -1; while (l < r) { int m = l + (r - l) / 2; if (nums[m] < lower) l = m + 1; else r = m; } return l; } int bs_upper(int *nums, int n, int left, int right, int upper) { int l = left, r = right; if (nums[l] > upper) return -1; while (l < r) { int m = r - (r - l) / 2; if (nums[m] > upper) r = m - 1; else l = m; } return l; } long long countFairPairs(int *nums, int numsSize, int lower, int upper) { int n = numsSize; long long ans = 0; qsort(nums, n, sizeof(int), cmp); for (int i = 0; i < n - 1; i++) { int left = bs_lower(nums, n, i + 1, n - 1, lower - nums[i]); int right = bs_upper(nums, n, i + 1, n - 1, upper - nums[i]); if (left != -1 && right != -1) ans += right - left + 1; } return ans; } int main() { int n1[] = {0, 1, 7, 4, 4, 5}, n2[] = {1, 7, 9, 2, 5}; printf("%lld\n", countFairPairs(n1, ARRAY_SIZE(n1), 3, 6)); // expect: 6 printf("%lld\n", countFairPairs(n2, ARRAY_SIZE(n2), 11, 11)); // expect: 1 }