Codeforces 934 C.A Twisty Movement-前缀和+后缀和+动态规划
1 second
256 megabytes
standard input
standard output
A dragon symbolizes wisdom, power and wealth. On Lunar New Year's Day, people model a dragon with bamboo strips and clothes, raise them with rods, and hold the rods high and low to resemble a flying dragon.
A performer holding the rod low is represented by a 1, while one holding it high is represented by a 2. Thus, the line of performers can be represented by a sequence a1, a2, ..., an.
Little Tommy is among them. He would like to choose an interval [l, r] (1 ≤ l ≤ r ≤ n), then reverse al, al + 1, ..., ar so that the length of the longest non-decreasing subsequence of the new sequence is maximum.
A non-decreasing subsequence is a sequence of indices p1, p2, ..., pk, such that p1 < p2 < ... < pk and ap1 ≤ ap2 ≤ ... ≤ apk. The length of the subsequence is k.
The first line contains an integer n (1 ≤ n ≤ 2000), denoting the length of the original sequence.
The second line contains n space-separated integers, describing the original sequence a1, a2, ..., an (1 ≤ ai ≤ 2, i = 1, 2, ..., n).
Print a single integer, which means the maximum possible length of the longest non-decreasing subsequence of the new sequence.
4
1 2 1 2
4
10
1 1 2 2 2 1 1 2 2 1
9
In the first example, after reversing [2, 3], the array will become [1, 1, 2, 2], where the length of the longest non-decreasing subsequence is 4.
In the second example, after reversing [3, 7], the array will become [1, 1, 1, 1, 2, 2, 2, 2, 2, 1], where the length of the longest non-decreasing subsequence is 9.
1 //C-用dp写
2 #include<iostream>
3 #include<cstring>
4 #include<cstdio>
5 #include<cmath>
6 #include<algorithm>
7 #include<cstdlib>
8 using namespace std;
9 const int maxn=2000+10;
10 int dp[maxn][maxn][2];
11 int p1[maxn],p2[maxn],a[maxn];
12 int main(){
13 int n;
14 scanf("%d",&n);
15 p1[0]=0;p2[n+1]=0;
16 for(int i=1;i<=n;i++){
17 scanf("%d",&a[i]);
18 p1[i]=p1[i-1];
19 if(a[i]==1)p1[i]++;
20 }
21 for(int j=n;j>=1;j--){
22 p2[j]=p2[j+1];
23 if(a[j]==2)p2[j]++;
24 }
25 int ans=-1;
26 for(int i=1;i<=n;i++){
27 for(int j=i;j<=n;j++){
28 if(a[j]==2)dp[i][j][1]=dp[i][j-1][1]+1;
29 else dp[i][j][1]=dp[i][j-1][1];
30 if(a[j]==1)dp[i][j][0]=max(dp[i][j-1][0],dp[i][j-1][1])+1;
31 else dp[i][j][0]=max(dp[i][j-1][0],dp[i][j-1][1]);
32 ans=max(p1[i-1]+p2[j+1]+dp[i][j][1],max(ans,p1[i-1]+p2[j+1]+dp[i][j][0]));
33 }
34 }
35 printf("%d\n",ans);
36 }
不用动态规划也可以直接模拟。
继续。
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