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.

Rust Solution

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);
}

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