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// 1218. Longest Arithmetic Subsequence of Given Difference
#include <stdio.h>
#include <stdlib.h>
/*
* given an integer array 'arr' and an integer 'difference', return the length
* of the longest subsequence in 'arr' which is an arithmetic sequence such that
* the difference between adjacent elements in the subsequence equals
* 'difference'. a subsequence is a sequence that can be derived from 'arr' by
* deleting some or no elements without changing the order of the remaining
* elements.
*/
int longestSubsequence(int *arr, int arr_size, int difference) {
int idx, max = 0, cnt[20001] = {0};
for (int i = 0; i < arr_size; i++) {
idx = 10000 + arr[i];
if (max < cnt[idx])
max = cnt[idx];
if (idx + difference >= 0 && idx + difference <= 2000)
cnt[idx + difference] = cnt[idx] + 1;
}
return max + 1;
}
int main() {
int a1[] = {1, 2, 3, 4}, a2[] = {1, 3, 5, 7},
a3[] = {1, 5, 7, 8, 5, 3, 4, 2, 1};
printf("%d\n", longestSubsequence(a1, 4, 1)); // expect: 4
printf("%d\n", longestSubsequence(a2, 4, 1)); // expect: 1
printf("%d\n", longestSubsequence(a3, 9, -2)); // expect: 4
}
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