题意：给定一个序列，求一个最长子序列，使得序列长度为奇数，并且前一半严格递增，后一半严格递减。
析：先正向和逆向分别求一次LIS，然后再枚举中间的那个数，找得最长的那个序列。
代码如下：
#pragma comment(linker, "/STACK:1024000000,1024000000")
#include <cstdio>
#include <string>
#include <cstdlib>
#include <cmath>
#include <iostream>
#include <cstring>
#include <set>
#include <queue>
#include <algorithm>
#include <vector>
#include <map>
#include <cctype>
#include <cmath>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#define debug() puts("++++");
#define freopenr freopen("in.txt", "r", stdin)
#define freopenw freopen("out.txt", "w", stdout)
using namespace std; typedef long long LL;
typedef pair<int, int> P;
const int INF = 0x3f3f3f3f;
const double inf = 0x3f3f3f3f3f3f;
const double PI = acos(-1.0);
const double eps = 1e-8;
const int maxn = 10000 + 5;
const int mod = 2000;
const int dr[] = {-1, 1, 0, 0};
const int dc[] = {0, 0, 1, -1};
const char *de[] = {"0000", "0001", "0010", "0011", "0100", "0101", "0110", "0111", "1000", "1001", "1010", "1011", "1100", "1101", "1110", "1111"};
int n, m;
const int mon[] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
const int monn[] = {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
inline bool is_in(int r, int c){
return r >= 0 && r < n && c >= 0 && c < m;
}
int dp1[maxn], dp2[maxn], dp[maxn];
int a[maxn]; int solve(){
int ans = 0;
for(int i = 1; i < n; ++i){
ans = max(ans, min(dp1[i], dp2[i])*2 + 1);
}
return ans;
} int main(){
while(scanf("%d", &n) == 1){
for(int i = 0; i < n; ++i) scanf("%d", a+i);
memset(dp, INF, sizeof dp);
for(int i = 0; i < n; ++i){
*lower_bound(dp, dp+n, a[i]) = a[i];
dp1[i] = lower_bound(dp, dp+i+1, INF) - dp - 1;
}
memset(dp, INF, sizeof dp);
for(int i = n-1; i >= 0; --i){
*lower_bound(dp, dp+n, a[i]) = a[i];
dp2[i] = lower_bound(dp, dp+n, INF) - dp - 1;
}
printf("%d\n", solve());
}
return 0;
}