[luoguP2659] 美丽的序列（单调栈）

传送门

单调栈大水题

l[i] 表示 i 能扩展到的左边

r[i] 表示 i 能扩展到的右边

——代码

 #include <cstdio>
 #include <iostream>
 #define LL long long

 const int MAXN = ;
 int n, t;
 LL ans, a[MAXN], s[MAXN], l[MAXN], r[MAXN];

 inline int read()
 {
     int x = , f = ;
     char ch = getchar();
     for(; !isdigit(ch); ch = getchar()) if(ch == '-') f = -;
     for(; isdigit(ch); ch = getchar()) x = (x << ) + (x << ) + ch - '';
     return x * f;
 }

 inline LL max(LL x, LL y)
 {
     return x > y ? x : y;
 }

 int main()
 {
     int i;
     n = read();
     for(i = ; i <= n; i++) a[i] = read();
     a[] = a[n + ] = -;
     for(i = ; i <= n + ; i++)
     {
         while(t && a[s[t]] > a[i]) r[s[t--]] = i;
         s[++t] = i;
     }
     t = ;
     for(i = n; i >= ; i--)
     {
         while(t && a[s[t]] > a[i]) l[s[t--]] = i;
         s[++t] = i;
     }
     for(i = ; i <= n; i++) ans = max(ans, a[i] * (r[i] - l[i] - ));
     printf("%lld\n", ans);
     return ;
 }