Longest ideal subsequence

Hash Table String Dynamic Programming

46.6% acceptance

Feb 25, 2026

1528

You are given a string s consisting of lowercase letters and an integer k. We call a string t ideal if the following conditions are satisfied: t is a subsequence of the string s. The absolute difference in the alphabet order of every two adjacent letters in t is less than or equal to k. Return the length of the longest ideal string. A subsequence is a string that can be derived from another string by deleting some or no characters without changing the order of the remaining characters. Note that the alphabet order is not cyclic. For example, the absolute difference in the alphabet order of 'a' and 'z' is 25, not 1.

Solution

Rust

Time O(n)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn longest_ideal_string(s: String, k: i32) -> i32 {
    let k = k as usize;
    let mut dp = [0i32; 26];
    for c in s.bytes() {
      let ci = (c - b'a') as usize;
      let lo = ci.saturating_sub(k);
      let hi = (ci + k).min(25);
      let best = *dp[lo..=hi].iter().max().unwrap();
      dp[ci] = best + 1;
    }
    *dp.iter().max().unwrap()
  }
}