744. Find Smallest Letter Greater Than Target

Given a list of sorted characters `letters` containing only lowercase letters, and given a target letter `target`, find the smallest element in the list that is larger than the given target.

Letters also wrap around. For example, if the target is `target = 'z'` and `letters = ['a', 'b']`, the answer is `'a'`.

Examples:

```Input:
letters = ["c", "f", "j"]
target = "a"
Output: "c"

Input:
letters = ["c", "f", "j"]
target = "c"
Output: "f"

Input:
letters = ["c", "f", "j"]
target = "d"
Output: "f"

Input:
letters = ["c", "f", "j"]
target = "g"
Output: "j"

Input:
letters = ["c", "f", "j"]
target = "j"
Output: "c"

Input:
letters = ["c", "f", "j"]
target = "k"
Output: "c"
```

Note:

1. `letters` has a length in range `[2, 10000]`.
2. `letters` consists of lowercase letters, and contains at least 2 unique letters.
3. `target` is a lowercase letter.

744. Find Smallest Letter Greater Than Target
``````struct Solution;

impl Solution {
fn next_greatest_letter(letters: Vec<char>, target: char) -> char {
let n = letters.len();
let mut l = 0;
let mut r = n;
while l < r {
let mid = l + (r - l) / 2;
if target < letters[mid] {
r = mid;
} else {
l = mid + 1;
}
}
letters[l % n]
}
}

#[test]
fn test() {
let letters: Vec<char> = vec!['c', 'f', 'j'];
let target = 'a';
let res = 'c';
assert_eq!(Solution::next_greatest_letter(letters, target), res);
let letters: Vec<char> = vec!['c', 'f', 'j'];
let target = 'c';
let res = 'f';
assert_eq!(Solution::next_greatest_letter(letters, target), res);
let letters: Vec<char> = vec!['c', 'f', 'j'];
let target = 'd';
let res = 'f';
assert_eq!(Solution::next_greatest_letter(letters, target), res);
let letters: Vec<char> = vec!['c', 'f', 'j'];
let target = 'g';
let res = 'j';
assert_eq!(Solution::next_greatest_letter(letters, target), res);
let letters: Vec<char> = vec!['c', 'f', 'j'];
let target = 'j';
let res = 'c';
assert_eq!(Solution::next_greatest_letter(letters, target), res);
let letters: Vec<char> = vec!['c', 'f', 'j'];
let target = 'k';
let res = 'c';
assert_eq!(Solution::next_greatest_letter(letters, target), res);
}
``````