分而治之O(nlogn)

可以优化到O(n)
每次找到分界点（大小写不全在的点）

小tip 大小写转换 ‘A’ ^ 32 = ‘ ‘, ‘a’ ^ 32 = ‘A’

class Solution {
public:
    string longestNiceSubstring(string s) {
        if (s.size() == 2) return s[0] == (s[1] ^ ' ') ? s : "";
        unordered_set<char> mp(s.begin(), s.end());
        string ans;
        vector<int> split;
        for (int i = 0; i < s.size(); i++) {
            char c = s[i];
            if (!(mp.count(c) && mp.count(c ^ ' '))) 
                split.push_back(i);
        }
        if (split.empty()) return s;
        split.insert(split.begin(), -1);
        split.push_back(s.size());
        for (int i = 0; i < split.size() - 1; i++) {
            int l = split[i] + 1, r = split[i + 1] - 1;
            if (r - l + 1 <= 1) continue;
            auto res = longestNiceSubstring(s.substr(l, r - l + 1));
            if (res.size() > ans.size()) ans = res;
        }
        return ans;
    }
};