2019icpc上海网络赛 F. Rhyme scheme

题目链接

预处理： $dp\left[i\right]\left[j\right]\left[k\right]$dp[i][j][k]表示i行诗，第j层，最大字母是k有多少个。
然后直接dfs找就行了。

#include<bits/stdc++.h>

#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define LL bign
#define pii pair<int,int>
#define SZ(x) (int)x.size()
#define all(x) x.begin(),x.end()

using namespace std;

const int MAXN=150;
struct bign
{
    int len, s[MAXN];
    bign ()
    {
        memset(s, 0, sizeof(s));
        len = 1;
    }
    bign (int num) { *this = num; }
    bign (const char *num) { *this = num; }
    bign operator = (const int num)
    {
        char s[MAXN];
        sprintf(s, "%d", num);
        *this = s;
        return *this;
    }
    bign operator = (const char *num)
    {
        for(int i = 0; num[i] == '0'; num++) ;  //去前导0
        len = strlen(num);
        for(int i = 0; i < len; i++) s[i] = num[len-i-1] - '0';
        return *this;
    }
    bign operator + (const bign &b) const //+
    {
        bign c;
        c.len = 0;
        for(int i = 0, g = 0; g || i < max(len, b.len); i++)
        {
            int x = g;
            if(i < len) x += s[i];
            if(i < b.len) x += b.s[i];
            c.s[c.len++] = x % 10;
            g = x / 10;
        }
        return c;
    }
    bign operator += (const bign &b)
    {
        *this = *this + b;
        return *this;
    }
    void clean()
    {
        while(len > 1 && !s[len-1]) len--;
    }
    bign operator * (const bign &b) //*
    {
        bign c;
        c.len = len + b.len;
        for(int i = 0; i < len; i++)
        {
            for(int j = 0; j < b.len; j++)
            {
                c.s[i+j] += s[i] * b.s[j];
            }
        }
        for(int i = 0; i < c.len; i++)
        {
            c.s[i+1] += c.s[i]/10;
            c.s[i] %= 10;
        }
        c.clean();
        return c;
    }
    bign operator *= (const bign &b)
    {
        *this = *this * b;
        return *this;
    }
    bign operator - (const bign &b)
    {
        bign c;
        c.len = 0;
        for(int i = 0, g = 0; i < len; i++)
        {
            int x = s[i] - g;
            if(i < b.len) x -= b.s[i];
            if(x >= 0) g = 0;
            else
            {
                g = 1;
                x += 10;
            }
            c.s[c.len++] = x;
        }
        c.clean();
        return c;
    }
    bign operator -= (const bign &b)
    {
        *this = *this - b;
        return *this;
    }
    bign operator / (const bign &b)
    {
        bign c, f = 0;
        for(int i = len-1; i >= 0; i--)
        {
            f = f*10;
            f.s[0] = s[i];
            while(f >= b)
            {
                f -= b;
                c.s[i]++;
            }
        }
        c.len = len;
        c.clean();
        return c;
    }
    bign operator /= (const bign &b)
    {
        *this  = *this / b;
        return *this;
    }
    bign operator % (const bign &b)
    {
        bign r = *this / b;
        r = *this - r*b;
        return r;
    }
    bign operator %= (const bign &b)
    {
        *this = *this % b;
        return *this;
    }
    bool operator < (const bign &b)
    {
        if(len != b.len) return len < b.len;
        for(int i = len-1; i >= 0; i--)
        {
            if(s[i] != b.s[i]) return s[i] < b.s[i];
        }
        return false;
    }
    bool operator > (const bign &b)
    {
        if(len != b.len) return len > b.len;
        for(int i = len-1; i >= 0; i--)
        {
            if(s[i] != b.s[i]) return s[i] > b.s[i];
        }
        return false;
    }
    bool operator == (const bign &b)
    {
        return !(*this > b) && !(*this < b);
    }
    bool operator != (const bign &b)
    {
        return !(*this == b);
    }
    bool operator <= (const bign &b)
    {
        return *this < b || *this == b;
    }
    bool operator >= (const bign &b)
    {
        return *this > b || *this == b;
    }
    string str() const
    {
        string res = "";
        for(int i = 0; i < len; i++) res = char(s[i]+'0') + res;
        return res;
    }
};
istream& operator >> (istream &in, bign &x)
{
    string s;
    in >> s;
    x = s.c_str();
    return in;
}
ostream& operator << (ostream &out, const bign &x)
{
    if (x.str()=="") out<<0;
    else out << x.str();
    return out;
}
const int N = 5e5 + 3;
int t,n,now,cnt;
LL k;
char a[33];
LL dp[30][30][30];
bign zero=0;
void dfs(int now,char x,int pos,char M='A'){
    if(dp[pos][now][M-'A']!=zero){return ;}
    dp[pos][now][M-'A']=0;
    if(now==pos){dp[pos][now][M-'A']=1;return;}
    for(char i='A';i<=M+1;i++){
        dfs(now+1,i,pos,max(M,i));
        dp[pos][now][M-'A']+=dp[pos][now+1][max(i,M)-'A'];
    }
}
void get(int now,char x,char M='A'){
    a[cnt++]=x;
    if(now==n)return ;
    LL pre=0;
    for(char i='A';i<=M+1;i++){
        if(pre+dp[n][now+1][max(M,i)-'A']<k){
            pre+=dp[n][now+1][max(M,i)-'A'];
        }else{
            k-=pre;
            get(now+1,i,max(M,i));
            break;
        }
    }
}
int main() {
    ios::sync_with_stdio(false);
    for(int i=1;i<=26;i++){
        for(int j=0;j<=26;j++){
            for(int k=0;k<26;k++)dp[i][j][k]=0;
        }
        dfs(1,'A',i);
    }
    for(cin>>t;t;t--){
        cin>>n>>k;
        cnt=0;
        get(1,'A');
        cout<<"Case #"<<++now<<": ";
        for(int i=0;i<n;i++)cout<<a[i];
        cout<<'\n';
    }
	return 0;
}