leetcode2309. The best English letters with both upper and lower case (simple, weekly)

Optimize 1： When traversing , Go back and forth , Traversing a[] and b[] All for true, be return
Optimize 2： Optimize the space to O(1)
Specific ideas ：
'A’65->90 97->122 122-65+1=58 Need to use 8 The bytes are long long Type to save status
add to ch：flag |= (ch - ‘A’)
Determine whether upper and lower case letters exist ：flag =（flag>>(‘a’-‘A’) & flag） by true
obtain ans：‘A’ + (flag Number of digits ：64 - __builtin_clzll(flag)) - 1

class Solution {
    
public:
    string greatestLetter(string s) {
    

        long long flag = 0;
        for (auto& ch : s) {
    
            flag |= (1ll << ch - 'A'); // 
        }
        flag &= (flag >> ('a' - 'A'));
        return flag ? string(1, 63 - __builtin_clzll(flag) + 'A') : "";
    }
};

It's easy to get wrong ：1ll Move left