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+// 1027. Longest Arithmetic Subsequence
+#include "leetcode.h"
+
+/*
+ * given an array 'nums' of integers, return the length of the longest
+ * arithmetic subsequence in 'nums'. note that
+ * - a subsequence is an array that can be derived from another array by
+ * deleting some or no elements without changing the order of the remaining
+ * elements.
+ * - a sequence 'seq' is arithmetic if 'seq[i + 1] - seq[i]' are all the same
+ * value (for '0 <= i < seq.len() - 1')
+ */
+
+class Solution {
+public:
+ int longestArithmeticSeqLength(vector<int> &nums) {
+ int n = nums.size(), ans = 0;
+ vvd dp(n, vector<int>(n, 1));
+ for (int i = 1; i < n - 1; ++i)
+ for (int j = i + 1; j < n; ++j) {
+ for (int k = i - 1; k >= 0; k--)
+ if (nums[i] - nums[k] == nums[j] - nums[i])
+ dp[i][j] = max(dp[i][j], dp[k][i] + 1);
+ ans = max(ans, dp[i][j]);
+ }
+ return ans + 1;
+ }
+};
+
+int main() {
+ Solution obj;
+ vector<int> n1 = {3, 6, 9, 12}, n2 = {9, 4, 7, 2, 10},
+ n3 = {20, 1, 15, 3, 10, 5, 8};
+ printf("%d\n", obj.longestArithmeticSeqLength(n1)); // expect: 4
+ printf("%d\n", obj.longestArithmeticSeqLength(n2)); // expect: 3
+ printf("%d\n", obj.longestArithmeticSeqLength(n3)); // expect: 4
+}