P3607 [USACO17JAN]Subsequence Reversal P

https://www.luogu.com.cn/record/86419056

Luogu/P3607/P3607.cpp

@@ -0,0 +1,57 @@
+#include <iostream>
+#include <algorithm>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 55;
+
+int n, a[N], f[N][N][N][N];
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n;
+
+    for (int i = 1; i <= n; i++) {
+        cin >> a[i];
+    }
+
+    for (int l = 1; l <= n; l++) {
+        for (int i = 1; i <= a[l]; i++) {
+            for (int j = a[l]; j <= 50; j++) {
+                f[l][l][i][j] = 1;
+            }
+        }
+    }
+
+    for (int len = 2; len <= n; len++) {
+        for (int l = 1, r = len; r <= n; l++, r++) {
+            for (int i = 1; i <= 50; i++) {
+                for (int j = i; j <= 50; j++) {
+                    f[l][r][i][j] = std::max({
+                        f[l][r][i][j],
+                        f[l + 1][r][i][j] + (a[l] == i),
+                        f[l][r - 1][i][j] + (a[r] == j),
+                        f[l + 1][r - 1][i][j] + (a[r] == i) + (a[l] == j),
+                    });
+
+                    f[l][r][i][j + 1] = std::max(f[l][r][i][j + 1], f[l][r][i][j]);
+                    f[l][r][i - 1][j] = std::max(f[l][r][i - 1][j], f[l][r][i][j]);
+                }
+            }
+
+            for (int j = 1; j <= 50; j++) {
+                for (int i = j; i; i--) {
+                    f[l][r][i - 1][j] = std::max(f[l][r][i - 1][j], f[l][r][i][j]);
+                }
+            }
+        }
+    }
+
+    cout << f[1][n][1][50] << endl;
+
+    return 0;
+}