#include <iostream>
#include <string>
using namespace std;
int main()
{
	string n;
	cin >> n;
	//如果为负数照常输出 
	if(n[0] == '-')
	{
		cout << n << endl;
	}
	//如果为0照常输出 
	else if(n == "0") cout << n << endl;
	//如果为不是负数加上负号照常输出 
	else
	{
		cout << "-" << n << endl;
	}
	return 0;
}

写了半天，试过了，也加了注释，点个赞在拿走吧

#include using namespace std;

int main() { int N; cin >> N; if(N>0){ cout<<-N; } else{ cout<<N; } return 0; }

#include #include using namespace std; int main() { string n; cin >> n; if(n[0] == '-') { cout << n << endl; } else if(n == "0") cout << n << endl; else { cout << "-" << n << endl; } return 0; }