[leetcode-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:
letters has a length in range [2, 10000].
letters consists of lowercase letters, and contains at least 2 unique letters.
target is a lowercase letter.

思路：

二分查找。

 char nextGreatestLetter(vector<char>& letters, char target) {
        if (letters.back() <= target) return letters.front();
        int low = 0, high = letters.size() - 1;
        while (low < high) {
            int mid = (low + high) / 2;
            if (target < letters[mid]) high = mid;
            else low = mid + 1;
        }
        return letters[low];
    }

参考：

https://leetcode.com/problems/find-smallest-letter-greater-than-target/discuss/110027