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authorjack <[email protected]>2024-06-14 13:13:15 -0600
committerjack <[email protected]>2024-06-14 13:13:15 -0600
commitd343a3b18dbaca9297d4f7b2280c4a16f0a5c8aa (patch)
tree96df922dae2a5073d9a50ea70c7aae6aa37e3ebe /count_num_fair_pairs.c
downloadleetcode-d343a3b18dbaca9297d4f7b2280c4a16f0a5c8aa.tar.gz
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+// 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
+}