CF897C Nephren gives a riddle

思路：

递归。

比赛的时候脑抽了len[]没算够，wa了几次。

实现：

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const string s = "What are you doing at the end of the world? Are you busy? Will you save us?";
const string t = "What are you doing while sending "";
const string v = ""? Are you busy? Will you send "";
const ll len_s = s.length();
const ll len_t = t.length();
const ll len_v = v.length();
ll len[100005];
char dfs(ll n, ll k)
{
    if (n == 0) 
    {
        if (k >= 1 && k <= len_s) return s[k - 1];
        else return '.';
    }
    else if (n >= 54)
    {
        if (k <= len_t) return t[k - 1];
        return dfs(n - 1, k - len_t);
    }
    else 
    {
        if (k <= len_t) return t[k - 1];
        else if (k > len_t && k <= len_t + len[n - 1])
            return dfs(n - 1, k - len_t);
        else if (k > len_t + len[n - 1] && k <= len_t + len[n - 1] + len_v)
            return v[k - len_t - len[n - 1] - 1];
        else if (k > len_t + len[n - 1] + len_v && k <= len_t + 2 * len[n - 1] + len_v)
            return dfs(n - 1, k - len_t - len[n - 1] - len_v);
        else if (k == len_t + 2 * len[n - 1] + len_v + 1)
            return '"';
        else if (k == len_t + 2 * len[n - 1] + len_v + 2)
            return '?';
        else return '.';
    }
}
int main()
{
    len[0] = (ll)len_s;
    for (int i = 1; i <= 53; i++)
    {
        len[i] = len[i - 1] * 2 + len_t + len_v + 2;
    }
    ll q, n, k;
    cin >> q;
    while (q--)
    {
        cin >> n >> k;
        cout << dfs(n, k);
    }
    return 0;
}