## 340. Longest Substring with At Most K Distinct Characters

Given a string `s` and an integer `k`, return the length of the longest substring of `s` that contains at most `k` distinct characters.

Example 1:

```Input: s = "eceba", k = 2
Output: 3
Explanation: The substring is "ece" with length 3.```

Example 2:

```Input: s = "aa", k = 1
Output: 2
Explanation: The substring is "aa" with length 2.
```

Constraints:

• `1 <= s.length <= 5 * 104`
• `0 <= k <= 50`

## Rust Solution

``````struct Solution;

use std::collections::HashMap;

impl Solution {
fn length_of_longest_substring_k_distinct(s: String, k: i32) -> i32 {
let k = k as usize;
let n = s.len();
let mut start = 0;
let mut end = 0;
let s: Vec<char> = s.chars().collect();
let mut count: HashMap<char, usize> = HashMap::new();
let mut res = 0;
while end < n {
*count.entry(s[end]).or_default() += 1;
end += 1;
while count
.values()
.fold(0, |acc, &v| acc + if v > 0 { 1 } else { 0 })
> k
{
*count.get_mut(&s[start]).unwrap() -= 1;
start += 1;
}
res = res.max(end - start);
}
res as i32
}
}

#[test]
fn test() {
let s = "eceba".to_string();
let k = 2;
let res = 3;
assert_eq!(Solution::length_of_longest_substring_k_distinct(s, k), res);
let s = "aa".to_string();
let k = 1;
let res = 2;
assert_eq!(Solution::length_of_longest_substring_k_distinct(s, k), res);
}
``````

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