LeetCode 744. 寻找比目标字母大的最小字母（二分查找）

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"

class Solution {
public:
    char nextGreatestLetter(vector<char>& letters, char target) {
        int l = 0, r = letters.size()-1, mid;
        while(l <= r)
        {
        	mid = l+((r-l)>>1);
        	if(letters[mid] > target)
        	{
        		if(mid != 0 && letters[mid-1] <= target)
        			return letters[mid];
        		else
        			r = mid-1;
        	}
        	else
        		l = mid+1;
        }
        return letters[0];
    }
};