HDU4850 Wow! Such String! —— 字符串构造

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=4850

代码如下：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <sstream>
#include <algorithm>
using namespace std;
#define pb push_back
#define mp make_pair
#define ms(a, b)  memset((a), (b), sizeof(a))
#define eps 0.0000001
typedef long long LL;
const int INF = 2e9;
const LL LNF = 9e18;
const int mod = 1e9+7;
const int maxn = 26*26*26*26;
int a[maxn+10];
int vis[26][26][26][26];


void solve()
{
    a[0] = a[1] = a[2] = 0;
    for(int i = 3; i<maxn+3; i++)
    {
        for(int j = 25; j>=0; j--)
        {
            if(vis[a[i-3]][a[i-2]][a[i-1]][j]==0)
            {
                vis[a[i-3]][a[i-2]][a[i-1]][j] = 1;
                a[i] = j;
                break;
            }
        }
    }
}

int main()
{
    int n;
    solve();
    while(scanf("%d",&n)!=EOF)
    {
        if(n>maxn+3)
            printf("Impossible");
        else
            for(int i = 0; i<n; i++)
                printf("%c",a[i]+'a');
        putchar('
');
    }
}