struct Solution;
use std::collections::HashMap;
impl Solution {
fn longest_arith_seq_length(a: Vec<i32>) -> i32 {
let mut res = 0;
let n = a.len();
let mut dp: Vec<HashMap<i32, i32>> = vec![HashMap::new(); n];
for i in 0..n {
for j in 0..i {
let diff = a[i] - a[j];
let len_j = *dp[j].entry(diff).or_default();
let len_i = dp[i].entry(diff).or_default();
*len_i = len_j + 1;
res = res.max(*len_i);
}
}
res + 1
}
}
#[test]
fn test() {
let a = vec![3, 6, 9, 12];
let res = 4;
assert_eq!(Solution::longest_arith_seq_length(a), res);
let a = vec![9, 4, 7, 2, 10];
let res = 3;
assert_eq!(Solution::longest_arith_seq_length(a), res);
let a = vec![20, 1, 15, 3, 10, 5, 8];
let res = 4;
assert_eq!(Solution::longest_arith_seq_length(a), res);
}