solution1

分别统计字符String的个数，并按照该顺序输出

#include<iostream>
#include<string>
#include<map>
using namespace std;
map<char, int> mp;
void handle(char c){
	if(mp.count(c)){
		cout << c;
		mp[c]--;
		if(mp[c] == 0) mp.erase(c);
	}
}
int main(){
	string s, m = "String";
	cin >> s;
	for(int i = 0; i < s.size(); i++){
		if(s[i] == 'S' || s[i] == 't' || s[i] == 'r' || s[i] == 'i' || s[i] == 'n' ||s[i] == 'g'){
			if(mp.count(s[i])) mp[s[i]]++;
			else mp[s[i]] = 1;
		}
	}
	while(mp.size()){
		for(int i = 0; i < m.size(); i++){
			handle(m[i]);
		}
	}
	return 0;
}

solution2

#include<iostream>
#include<string>
using namespace std;
int main(){
	string s, m = "String";
	int flag[150] = {0}, ans = 1;
	cin >> s;
	for(int i = 0; i < s.size(); i++){
		flag[s[i]]++;
	}
	while(ans){//上一轮有输出，则还有必要尝试下一轮
		ans = 0;//默认本轮没有输出，则已经全部输出无需继续
		for(int i = 0; i < m.size(); i++){
			if(flag[m[i]] > 0){
				cout << m[i];
				flag[m[i]]--;
				ans = 1;
			}
		}
	}
	return 0;
}