字符串与日期处理-4

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<cmath>
#include<string>

using namespace std;
const int maxn=10010;
const int N=10;

char s[500001];

int main(){
	
	int n;
	cin>>n;
	
	int len=0;
	for(int i=1;i<=n;i++){
		
		strcat(s+len+1,s);
		s[len]='A'+i-1;
		
		len=len*2+1//len=strlen(s); 降低时间复杂度 
	}
	
	printf("%s\n",s);

	return 0;
}