E - A Twisty Movement
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.
Input
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).
Output
Print a single integer, which means the maximum possible length of the longest non-decreasing subsequence of the new sequence.
Examples
4
1 2 1 2
4
10
1 1 2 2 2 1 1 2 2 1
9
Note
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.
分成堆:
1 2 1 2
就交换中间的2 1 就行
所以找前面的1 2和后面的1 2的 最长不降就行
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define inf 2147483647
const ll INF = 0x3f3f3f3f3f3f3f3fll;
#define ri register int
template <class T> inline T min(T a, T b, T c)
{
return min(min(a, b), c);
}
template <class T> inline T max(T a, T b, T c)
{
return max(max(a, b), c);
}
template <class T> inline T min(T a, T b, T c, T d)
{
return min(min(a, b), min(c, d));
}
template <class T> inline T max(T a, T b, T c, T d)
{
return max(max(a, b), max(c, d));
}
#define scanf1(x) scanf("%d", &x)
#define scanf2(x, y) scanf("%d%d", &x, &y)
#define scanf3(x, y, z) scanf("%d%d%d", &x, &y, &z)
#define scanf4(x, y, z, X) scanf("%d%d%d%d", &x, &y, &z, &X)
#define pi acos(-1)
#define me(x, y) memset(x, y, sizeof(x));
#define For(i, a, b) for (int i = a; i <= b; i++)
#define FFor(i, a, b) for (int i = a; i >= b; i--)
#define bug printf("***********\n");
#define mp make_pair
#define pb push_back
const int N = ;
// name*******************************
int f1[N];
int f2[N];
int pre[N];
int nxt[N];
int n;
int a[N];
int ans=;
// function****************************** //***************************************
int main()
{
// ios::sync_with_stdio(0);
// cin.tie(0);
// freopen("test.txt", "r", stdin);
// freopen("outout.txt","w",stdout);
cin>>n;
For(i,,n)
cin>>a[i]; For(i,,n)
{
f1[i]=;
For(j,,i-)
if(a[i]>=a[j])
f1[i]=max(f1[i],f1[j]+);
pre[i]=max(pre[i-],f1[i]);
} FFor(i,n,)
{
f2[i]=;
For(j,i+,n)
if(a[j]>=a[i])
f2[i]=max(f2[i],f2[j]+);
nxt[i]=max(nxt[i+],f2[i]);
} For(i,,n)
{
ans=max(ans,nxt[i+]+pre[i]);
}
cout<<ans; return ;
}
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