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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
}
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