Leetcode744寻找比目标字母大的最小字母

题目

给定一个只包含小写字母的有序数组letters 和一个目标字母 target，寻找有序数组里面比目标字母大的最小字母。

数组里字母的顺序是循环的。举个例子，如果目标字母target = 'z' 并且有序数组为 letters = ['a', 'b']，则答案返回 'a'。

输入:
letters = ["c", "f", "j"]
target = "a"
输出: "c"

输入:
letters = ["c", "f", "j"]
target = "c"
输出: "f"

输入:
letters = ["c", "f", "j"]
target = "d"
输出: "f"

输入:
letters = ["c", "f", "j"]
target = "g"
输出: "j"

输入:
letters = ["c", "f", "j"]
target = "j"
输出: "c"

输入:
letters = ["c", "f", "j"]
target = "k"
输出: "c"

题解

本题采用二分查找来求解，转换为查找左边界的二分查找问题。将目标值加一再在原数组中进行左边界的二分查找，相当于求解大于给定元素的最小元素。

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