2309. The best English letters with both appropriates and lowercases One question daily

2309. The best English letters with both appropriates and lowercases

Thought：

1. Hash table：The first thing to think of the question is to solve it with a dictionary。But find that you can directly traverse the alphabet，fromZStart looking for，See if the applause exists，Just return directly。
   
   Running time exceeds99.9%
2. Bit operation：We can use two integers mask1 and mask2 Record string separately s 中出现的小写字母and大写字母，in mask1 First i
   Position representation i Does a lowercase letter appear，and mask2 First i Position representation i Whether an uppercase letter appears。

Then we will mask1 and mask2 Perform and calculate，The results obtained mask First i Position representation i Whether the lethals of the letter appear at the same time。

As long as you get mask The highest level of binary representation 1 s position，Convert it to the corresponding capital letter。If all binary positions are not 1，Explain that there is no letter that appears at the same time,，Return to an empty string。

author：ylb
Link：https://leetcode.cn/problems/greatest-english-letter-in-upper-and-lower-case/solutions/2077636/by-lcbin-zbg0/
source：Deduction（LeetCode）
著作权归author所有。商业转载请联系author获得授权，Non -commercial reprint Please indicate the source。

Code：

Traversing the alphabet

class Solution:
    def greatestLetter(self, s: str) -> str:
        for i in range(90, 64, -1):
            if chr(i) in s and chr(i+32) in s:
                return chr(i)
        return ""

Hash table

class Solution {
public:
    string greatestLetter(string s) {
        unordered_set<char> strin(s.begin(),s.end());
        for(char c = 'Z'; c >= 'A'; --c){
            if(strin.count(c) && strin.count(char(c+32))){
                return string(1,c);
            }
        }
        return "";
    }
};

Bit operation

class Solution:
    def greatestLetter(self, s: str) -> str:
        # We can use two integers mask1 and mask2
        # Record string separately s 中出现的小写字母and大写字母，in mask1 First i Position representation i Does a lowercase letter appear，
            # and mask2 First i Position representation i Whether an uppercase letter appears。
        mask1 = mask2 = 0
        for i in s:
            if i.islower():
                mask1 |= 1 << (ord(i) - ord("a"))
            else:
                mask2 |= 1 << (ord(i) - ord("A"))
        mask = mask1 & mask2
        return chr(mask.bit_length() - 1 + ord("A")) if mask else ""