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// 1218. Longest Arithmetic Subsequence of Given Difference
#include "leetcode.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.
*/
class Solution {
public:
int longestSubsequence(vector<int> &arr, int difference) {
int ans = 1;
unordered_map<int, int> max;
for (int i = 0; i < arr.size(); i++)
max[arr[i]] = max[arr[i] - difference] + 1;
for (auto i : max)
ans = std::max(ans, i.second);
return ans;
}
};
int main() {
Solution obj;
vector<int> a1 = {1, 2, 3, 4}, a2 = {1, 3, 5, 7},
a3 = {1, 5, 7, 8, 5, 3, 4, 2, 1};
printf("%d\n", obj.longestSubsequence(a1, 1)); // expect: 4
printf("%d\n", obj.longestSubsequence(a2, 1)); // expect: 1
printf("%d\n", obj.longestSubsequence(a3, -2)); // expect: 4
}
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