P2467-[SDOI2010]地精部落

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 typedef long long ll;
 4 typedef double db;
 5 #define INF 0x3f3f3f3f
 6 #define _for(i,a,b) for(int i = (a);i < b;i ++)
 7 #define _rep(i,a,b) for(int i = (a);i > b;i --)
 8 inline ll read()
 9 {
10     ll ans = 0;
11     char ch = getchar(), last = ' ';
12     while(!isdigit(ch)) last = ch, ch = getchar();
13     while(isdigit(ch)) ans = (ans << 1) + (ans << 3) + ch - '0', ch = getchar();
14     if(last == '-') ans = -ans;
15     return ans;
16 }
17 inline void write(ll x)
18 {
19     if(x < 0) x = -x, putchar('-');
20     if(x >= 10) write(x / 10);
21     putchar(x % 10 + '0');
22 }
23 int n,mod; 
24 int dp[2][6002];
25 int main()
26 {
27     n = read(), mod = read();
28     dp[0][2] = 1;
29     _for(i,3,n+1)
30         _for(j,2,i+1)
31             dp[i&1][j] = (dp[i&1][j-1]+dp[(i-1)&1][i-j+1])%mod;
32     int rnt = 0;
33     _for(i,2,n+1)
34         rnt = (rnt+dp[n&1][i])%mod;
35     write((rnt<<1)%mod);
36     return 0;
37 }