【C++】时间复杂度O(n)

#include<string>
using namespace std;
#define MAX 1000

int main() {


	
	string s="start";
	while (s != "") {
		getline(cin, s);
		string mem = "";
		for (int i = 0; i < 26; i++) {
			for (int j = 0; j < s.size(); j++) {
				if ((s[j] - 'a' == i) || (s[j] - 'A' == i)) {
					mem += s[j];//把所有英文字母存到一个容器中
				}
			}
		}
		int index = 0;
		for (int i = 0; i < s.size(); i++) {
			if ((s[i] >= 'a' & s[i] <= 'z') || (s[i] >= 'A' & s[i] <= 'Z')) {
				s[i] = mem[index++];
			}
			if (index == mem.size())break;
		}

		cout << s << endl;

	}
	return 0;
}