代码:

#include <bits/stdc++.h>
using namespace std;
int main()
{
	string s;
	cin>>s;
	for(int i; i<strlen(s); i++)
	{
		if (s[i] == 'A') cout<<'T';
		else if(s[i] =='T') cout<<'A';
		if(s[i] == 'G') cout<<'C';
		else if(s[i] =='C') cout<<'G';
	}
	return 0;
}

[Error] cannot convert 'std::string' {aka 'std::__cxx11::basic_string'} to 'const char*'