// 1218. Longest Arithmetic Subsequence of Given Difference #include #include /* * 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 }