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#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
using namespace std;
 
void solve(string &s, int n)
{
    if(s.size() > 0)
    {
        solve(s.substr(0, n - 1), n - 1);
        cout << string(n - 1, ' ') << s << endl;
    }
}
 
int main(){
    int n;
    cin >> n;
    solve(string(n, '#'), n);
    return 0;
}

prog.cpp: In function 'void solve(std::string&, int)':
prog.cpp:13:23: error: invalid initialization of non-const reference of type 'std::string& {aka std::basic_string<char>&}' from an rvalue of type 'std::basic_string<char>'
         solve(s.substr(0, n - 1), n - 1);
                       ^
prog.cpp:9:6: note:   initializing argument 1 of 'void solve(std::string&, int)'
 void solve(string &s, int n)
      ^
prog.cpp: In function 'int main()':
prog.cpp:21:28: error: invalid initialization of non-const reference of type 'std::string& {aka std::basic_string<char>&}' from an rvalue of type 'std::string {aka std::basic_string<char>}'
     solve(string(n, '#'), n);
                            ^
prog.cpp:9:6: note:   initializing argument 1 of 'void solve(std::string&, int)'
 void solve(string &s, int n)
      ^

Standard output is empty