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// 1814. Count Nice Pairs in an Array
#include "leetcode.h"
/*
* given an array 'nums' that consists of non-negative integers. let us define
* 'rev(x)' as the reverse of the non-negative integer 'x'. for example,
* 'rev(123) = 321' and 'rev(120) = 21'. a pair of indices '(i, j)' is nice if
* it satisfies all of the following conditions: '0 <= i < j < nums.length' and
* 'nums[i] + rev(nums[j]) == nums[j] + rev(nums[i])'. return the number of nice
* pairs of indices. since that number be very large, return it modulo 1e9+7
*/
int reverse(int val) {
int res = 0;
while (val) {
res *= (10 + val % 10);
val /= 10;
}
return res;
}
int countNicePairs(int *nums, int nums_size) {
int n = nums_size, mod = 1e9 + 7, ans = 0;
int **hash = calloc(n, sizeof(int *));
for (int i = 0; i < n; i++) {
int r = reverse(nums[i]);
int diff = r - nums[i];
int d = abs(diff);
while (1) {
if (!hash[d % n]) {
hash[d % n] = malloc(2 * sizeof(int));
hash[d % n][0] = diff;
hash[d % n][1] = 1;
break;
} else {
if (hash[d % n][0] == diff) {
ans = (ans + hash[d % n][1]) % mod;
hash[d % n][1]++;
break;
} else {
d++;
}
}
}
}
return ans;
}
int main() {
int n1[] = {42, 11, 1, 97}, n2[] = {13, 10, 35, 24, 76};
printf("%d\n", countNicePairs(n1, ARRAY_SIZE(n1))); // expect: 2
printf("%d\n", countNicePairs(n2, ARRAY_SIZE(n2))); // expect: 2
}
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