P8446 虹色的北斗七星

https://www.luogu.com.cn/record/80990615

Luogu/P8446/P8446.cpp

@@ -0,0 +1,63 @@
+#include <cstdio>
+#include <cctype>
+#include <cstring>
+#include <stack>
+
+const int N = 4e6 + 5;
+
+template <typename T>
+void read(T &x) {
+    x = 0;
+    T sig = 1;
+    char c = getchar();
+
+    for (; !isdigit(c); c = getchar()) {
+        if (c == '-') sig = -1;
+    }
+
+    for (; isdigit(c); c = getchar()) {
+        x = (x << 1) + (x << 3) + c - '0';
+    }
+
+    x *= sig;
+}
+
+int n, a[N];
+long long ans_min[N], ans_max[N], ans = -1;
+std::stack<long long> st_min, st_max;
+
+int main() {
+    read(n);
+
+    for (int i = 1; i <= n; i++) {
+        read(a[i]);
+    }
+
+    memset(ans_min, 0x3f, sizeof(ans_min));
+
+    for (int i = 1; i <= n; i++) {
+        while (!st_max.empty() && a[st_max.top()] > a[i]) {
+            auto x = ans_max[st_max.top()];
+            ans = std::max(ans, x - a[i] - i - 1);
+            ans_max[i] = std::max(ans_max[i], x);
+            st_max.pop();
+        }
+        st_max.push(i);
+        ans_max[i] = std::max(ans_max[i], static_cast<long long>(a[i]) + i);
+    }
+
+    for (int i = 1; i <= n; i++) {
+        while (!st_min.empty() && a[st_min.top()] < a[i]) {
+            auto x = ans_min[st_min.top()];
+            ans = std::max(ans, static_cast<long long>(a[i]) - i - x - 1);
+            ans_min[i] = std::min(ans_min[i], x);
+            st_min.pop();
+        }
+        st_min.push(i);
+        ans_min[i] = std::min(ans_min[i], static_cast<long long>(a[i]) - i);
+    }
+
+    printf("%lld\n", ans);
+
+    return 0;
+}