class Solution {
public:
string sortString(string s) {
vector<int> num(26);
for (char &ch : s) {
num[ch - 'a']++;
}
string ret;
while (ret.length() < s.length()) {
for (int i = 0; i < 26; i++) {
if (num[i]) {
ret.push_back(i + 'a');
num[i]--;
}
}
for (int i = 25; i >= 0; i--) {
if (num[i]) {
ret.push_back(i + 'a');
num[i]--;
}
}
}
return ret;
}
};