URAL 1260 Nudnik Photographer DFS DP

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：这个题可以先dfs深搜下，规律dp

dfs：

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 #define S second
 4 typedef long long ll;
 5 const int INF = 0x3f3f3f3f;
 6 const int M = 1e5+3;
 7
 8 int s;
 9 int cnt = 0;
10 bool vis[60];
11 void dfs( int last, int num )   {
12     if( num == s )  {
13         cnt++;
14         return ;
15     }
16     for( int i=2; i<s+1; i++ )
17         if( abs(last-i)<=2 && !vis[i] ) {
18             vis[i] = true;
19             dfs( i, num+1 );
20             vis[i] = false;
21         }
22 }
23 int main()  {
24     while( ~scanf("%d", &s ) )   {
25         memset( vis, false, sizeof(vis) );
26         cnt = 0;
27         vis[1] = true;
28         dfs( 1, 1 );
29         printf("%d\n", cnt );
30     }
31     return 0;
32 }

DP：

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 #define F first
 4 #define S second
 5 typedef unsigned long long ll;
 6 const int INF = 0x3f3f3f3f;
 7 const int M = 1e5+3;
 8
 9 int s;
10 ll dp[60];
11 void pre()  {
12     memset( dp, 0, sizeof(dp) );
13     dp[1] = 1;
14     dp[2] = 1;
15     dp[3] = 2;
16     dp[4] = 4;
17     for( int i=5; i<=55; i++ )
18         dp[i] = dp[i-1]+dp[i-3]+1;
19 }
20 int main()  {
21     pre();
22     while( ~scanf("%d", &s ) )   {
23         printf("%I64d\n", dp[s] );
24     }
25     return 0;
26 }