C++ :

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int main()

{
	char a[256];
	int len;
	int i;
		
	fgets(a,255,stdin);
	len=strlen(a);//计算字符串长度
	for(i=0;i<len;i++)//输出配对碱基
	{
		if(a[i]=='A')	cout<<"T";
		if(a[i]=='T')	cout<<"A";
		if(a[i]=='G')	cout<<"C";
		if(a[i]=='C')	cout<<"G";
	}
	cout<<endl;
	return 0;
}