【BZOJ1609】[Usaco2008 Feb]Eating Together麻烦的聚餐【最长不下降子序列】

【题目链接】

正反做一次最长不下降子序列，显然答案是n减去最长不下降子序列的长度。

/* Footprints In The Blood Soaked Snow */
#include <cstdio>
#include <algorithm>

using namespace std;

const int maxn = 30005, inf = 0x3f3f3f3f;

int n, num[maxn], dp[maxn], g[maxn];

inline int iread() {
int f = 1, x = 0; char ch = getchar();
for(; ch < '0' || ch > '9'; ch = getchar()) f = ch == '-' ? -1 : 1;
for(; ch >= '0' && ch <= '9'; ch = getchar()) x = x * 10 + ch - '0';
return f * x;
}

inline int calc() {
int ans = 0;
for(int i = 1; i <= n; i++) g[i] = inf;
for(int i = 1; i <= n; i++) {
int k = upper_bound(g + 1, g + 1 + n, num[i]) - g;
dp[i] = k;
g[k] = num[i];
ans = max(ans, dp[i]);
}
return n - ans;
}

int main() {
n = iread();
for(int i = 1; i <= n; i++) num[i] = iread();

int ans = n;
ans = min(ans, calc());
reverse(num + 1, num + 1 + n);
ans = min(ans, calc());

printf("%d\n", ans);
return 0;
}