## 1698. Number of Distinct Substrings in a String

Given a string `s`, return the number of distinct substrings of `s`.

A substring of a string is obtained by deleting any number of characters (possibly zero) from the front of the string and any number (possibly zero) from the back of the string.

Example 1:

```Input: s = "aabbaba"
Output: 21
Explanation: The set of distinct strings is ["a","b","aa","bb","ab","ba","aab","abb","bba","aba","aabb","abba","bbab","baba","aabba","abbab","bbaba","aabbab","abbaba","aabbaba"]
```

Example 2:

```Input: s = "abcdefg"
Output: 28
```

Constraints:

• `1 <= s.length <= 500`
• `s` consists of lowercase English letters.

Follow up: Can you solve this problem in `O(n)` time complexity?

## Rust Solution

``````struct Solution;

#[derive(Default)]
struct Trie {
children: [Option<Box<Trie>>; 26],
}

impl Solution {
fn count_distinct(s: String) -> i32 {
let n = s.len();
let s: Vec<u8> = s.bytes().collect();
let mut root: Trie = Trie::default();
let mut res = 0;
for i in 0..n {
let mut cur = &mut root;
for j in i..n {
if cur.children[(s[j] - b'a') as usize].is_none() {
cur.children[(s[j] - b'a') as usize] = Some(Box::new(Trie::default()));
res += 1;
}
cur = cur.children[(s[j] - b'a') as usize].as_mut().unwrap();
}
}
res
}
}

#[test]
fn test() {
let s = "aabbaba".to_string();
let res = 21;
assert_eq!(Solution::count_distinct(s), res);
let s = "abcdefg".to_string();
let res = 28;
assert_eq!(Solution::count_distinct(s), res);
}
``````

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