LeetCode 1027. Longest Arithmetic Sequence

Problem Statement

use std::collections::HashMap;

impl Solution {
    pub fn longest_arith_seq_length(a: Vec<i32>) -> i32 {
        let mut dp: HashMap<i32, HashMap<usize, i32>> = HashMap::new();
        let mut ans = 2;
        for i in 0..a.len() {
            for j in i + 1..a.len() {
                let d = a[j] - a[i];
                dp.entry(d).or_insert(HashMap::new());
                let val = dp.get_mut(&d).unwrap();
                let insert_val ;
                if val.contains_key(&i) {
                    insert_val = val.get(&i).unwrap().clone() + 1;
                } else {
                    insert_val = 2;
                }
                val.insert(j, insert_val);
                ans = std::cmp::max(ans, insert_val);
            }
        }

        return ans;
    }
}