leetcode

0 class Solution {

1 public:

2 int longestArithSeqLength(vector<int> &nums) {

3 unordered_map<int, vector<int>> um;

4 int res = 2;

5

6 for (int i = 0; i < nums.size(); i++)

7 um[nums[i]].push_back(i);

8 for (const auto &[num1, vec1] : um) {

9 res = max(res, (int)vec1.size());

10 for (const auto &[num2, vec2] : um) {

11 if (num1 == num2) continue;

12

13 auto it = lower_bound(vec2.begin(), vec2.end(), vec1.front() + 1);

14 if (it == vec2.end()) continue;

15

16 int diff = num2 - num1, crnt = *it, count = 2, next;

17 while (true) {

18 if (!um.count(next = nums[crnt] + diff)) break;

19 auto it = lower_bound(um[next].begin(), um[next].end(), crnt + 1);

20 if (it == um[next].end()) break;

21 crnt = *it, count++;

22 }

23 res = max(res, count);

24 }

25 }

26 return res;

27 }

28 };