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 = 10005; // name******************************* int f1[N]; int f2[N]; int pre[N]; int nxt[N]; int n; int a[N]; int ans=0; // 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,1,n) cin>>a[i]; For(i,1,n) { f1[i]=1; For(j,1,i-1) if(a[i]>=a[j]) f1[i]=max(f1[i],f1[j]+1); pre[i]=max(pre[i-1],f1[i]); } FFor(i,n,1) { f2[i]=1; For(j,i+1,n) if(a[j]>=a[i]) f2[i]=max(f2[i],f2[j]+1); nxt[i]=max(nxt[i+1],f2[i]); } For(i,1,n) { ans=max(ans,nxt[i+1]+pre[i]); } cout<<ans; return 0; }
