744. Find Smallest Letter Greater Than Target #

题目 #

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.

题目大意 #

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’。

注:

1. letters长度范围在[2, 10000]区间内。
2. letters 仅由小写字母组成，最少包含两个不同的字母。
3. 目标字母target 是一个小写字母。

解题思路 #

• 给出一个字节数组，在这个字节数组中查找在 target 后面的第一个字母。数组是环形的。
• 这一题也是二分搜索的题目，先在数组里面查找 target，如果找到了，取这个字母的后一个字母。如果没有找到，就取 low 下标的那个字母。注意数组是环形的，所以最后结果需要对下标取余。

代码 #

package leetcode

func nextGreatestLetter(letters []byte, target byte) byte {
	low, high := 0, len(letters)-1
	for low <= high {
		mid := low + (high-low)>>1
		if letters[mid] > target {
			high = mid - 1
		} else {
			low = mid + 1
		}
	}
	find := letters[low%len(letters)]
	if find <= target {
		return letters[0]
	}
	return find
}