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'.

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.

LeetCode744题，找到比目标字符大的第一个字符。此题Accepted代码如下：

class Solution {
    public char nextGreatestLetter(char[] letters, char target) {
        char a = 0;
        for (int i = 0; i < letters.length; i++) {
            if (letters[i] > target) {
                a = letters[i];
                break;
            } else {
                a = letters[0];
            }
        }
        return a;
    }
}

此题逻辑不复杂，但是由于我开始第3行只写了char c，运行的时候总是报未初始化。而后我又尝试了char c = '' char c = null，发现均不可行。后仔细思考，char不是引用类型，不可能和null有关系，写成以上或char a = 'a'均可。