URAL 2018. The Debut Album (dp)
2017-04-30 21:39
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Description
Pop-group “Pink elephant” entered on recording their debut album. In fact they have only two songs: “My love” and “I miss you”, but each of them has a large number of remixes.The producer of the group said that the album should consist of n remixes. On second thoughts the musicians decided that the album will be of interest only if there are no more than a remixes on “My love” in a row and no more than b remixes on “I miss you” in a row. Otherwise, there is a risk that even the most devoted fans won’t listen to the disk up to the end.
How many different variants to record the album of interest from n remixes exist? A variant is a sequence of integers 1 and 2, where ones denote remixes on “My love” and twos denote remixes on “I miss you”. Two variants are considered different if for some i in one variant at i-th place stands one and in another variant at the same place stands two.
Input
The only line contains integers n, a, b (1 ≤ a, b ≤ 300; max(a,b) + 1 ≤ n ≤ 50 000).Output
Output the number of different record variants modulo 109+7.Sample input
3 2 1
Sample output
4
题意
一个长度为n的数列,其中
1连续的个数不能超过
a,
2连续的个数不能超过
b,问总共有多少个这样的数列。
思路
dp[i][k]代表长度为
i的数列,以
k结尾并满足题意的个数。
则有:
dp[i][k]+=dp[i-j][k^1]
其中
dp[i-j][k^1]代表当前点
i之前最远距离为
a||b的那一点与当前点不同的情况,因为在这个范围以内都满足题意。
AC 代码
#include<bits/stdc++.h> using namespace std; const int mod=1e9+7; int dp[51000][2]; int main() { int n,a,b; cin>>n>>a>>b; dp[0][0]=dp[0][1]=1; for(int i=1; i<=n; i++) { for(int j=1; j<=min(i,a); j++) dp[i][0]=(dp[i][0]+dp[i-j][1])%mod; for(int j=1; j<=min(i,b); j++) dp[i][1]=(dp[i][1]+dp[i-j][0])%mod; } cout<<(dp [0]+dp [1])%mod; return 0; }
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