744. Find Smallest Letter Greater Than Target

Description

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.

Solution

Binary search

class Solution {
    public char nextGreatestLetter(char[] letters, char target) {
        int left = 0;
        int right = letters.length - 1;
        
        while (left <= right) {
            int mid = left + (right - left) / 2;
            
            if (letters[mid] <= target) {
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }
        
        return left < letters.length ? letters[left] : letters[0];
    }
}