## 159. Longest Substring with At Most Two Distinct Characters

Given a string s , find the length of the longest substring t  that contains at most 2 distinct characters.

Example 1:

```Input: "eceba"
Output: 3
Explanation: t is "ece" which its length is 3.
```

Example 2:

```Input: "ccaabbb"
Output: 5
Explanation: t is "aabbb" which its length is 5.
```

## Rust Solution

``````struct Solution;

impl Solution {
fn length_of_longest_substring_two_distinct(s: String) -> i32 {
let s: Vec<char> = s.chars().collect();
let mut count: Vec<usize> = vec![0; 256];
let mut start = 0;
let mut end = 0;
let mut res = 0;
let n = s.len();
let mut sum = 0;
while end < n {
if sum <= 2 {
count[s[end] as usize] += 1;
if count[s[end] as usize] == 1 {
sum += 1;
}
end += 1;
} else {
count[s[start] as usize] -= 1;
if count[s[start] as usize] == 0 {
sum -= 1;
}
start += 1;
}
if sum <= 2 {
res = res.max(end - start);
}
}
res as i32
}
}

#[test]
fn test() {
let s = "eceba".to_string();
let res = 3;
assert_eq!(Solution::length_of_longest_substring_two_distinct(s), res);
let s = "ccaabbb".to_string();
let res = 5;
assert_eq!(Solution::length_of_longest_substring_two_distinct(s), res);
}
``````

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