HDU-4850 Wow! Such String! （构造）

# include<iostream>
# include<cstdio>
# include<map>
# include<string>
# include<cstring>
# include<algorithm>
using namespace std;
const int N=26*26*26*26+3;
int vis[26][26][26][26],n;
char ans[N+10];
void get_ans()
{
    memset(vis,0,sizeof(vis));
    int pos=0;
    for(char a=‘a‘;a<=‘z‘;++a)
        ans[pos++]=a,ans[pos++]=a,ans[pos++]=a,ans[pos++]=a;
    for(int i=3;i<104;++i)
        vis[ans[i-3]-‘a‘][ans[i-2]-‘a‘][ans[i-1]-‘a‘][ans[i]-‘a‘]=1;
    char a=‘z‘;
    while(pos<456979){
        int cnt=0;
        for(int c=a+1;cnt<2;++c){
            if(c>‘z‘){
                c=‘a‘;
                ++cnt;
            }
            if(vis[ans[pos-3]-‘a‘][ans[pos-2]-‘a‘][ans[pos-1]-‘a‘][c-‘a‘])
                continue;
            vis[ans[pos-3]-‘a‘][ans[pos-2]-‘a‘][ans[pos-1]-‘a‘][c-‘a‘]=1;
            ans[pos++]=c;
            a=c;
            break;
        }
    }
}
int main()
{
    get_ans();
    while(~scanf("%d",&n))
    {
        if(n>N){
            printf("Impossible\n");
            continue;
        }
        for(int i=0;i<n;++i)
            printf("%c",ans[i]);
        printf("\n");
    }
    return 0;
}