744. Find Smallest Letter Greater Than Target

Given a list of sorted characters letterscontaining 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.

 

题目是变种版的二分查找,返回值字符数组的索引用取余搞定目标比所有字符大这个问题。

char nextGreatestLetter(char* letters, int lettersSize, char target) {
        int low=0, high=lettersSize;
        while(low<high){
            int mid=low+(high-low)/2;
            if(letters[mid]>target)    high=mid;//如果比目标大，就往左边找
            else    low=mid+1;               //如果比目标小，就往右边推
        }
        /*l
        因为etters数组索引范围是0-lettersSize-1,当low大于lettersSize-1的情况下，
        说明字符数组里面不存在比目标大的字符，此时low==lettersSize,对lettersSize进行取余,
        返回字符数组第一个元素。
        */
        return letters[low%lettersSize];
}