[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

 1 #include <cstdio>
 2 #include <iostream>
 3 #define LL long long
 4
 5 const int MAXN = 2000002;
 6 int n, t;
 7 LL ans, a[MAXN], s[MAXN], l[MAXN], r[MAXN];
 8
 9 inline int read()
10 {
11     int x = 0, f = 1;
12     char ch = getchar();
13     for(; !isdigit(ch); ch = getchar()) if(ch == ‘-‘) f = -1;
14     for(; isdigit(ch); ch = getchar()) x = (x << 1) + (x << 3) + ch - ‘0‘;
15     return x * f;
16 }
17
18 inline LL max(LL x, LL y)
19 {
20     return x > y ? x : y;
21 }
22
23 int main()
24 {
25     int i;
26     n = read();
27     for(i = 1; i <= n; i++) a[i] = read();
28     a[0] = a[n + 1] = -1;
29     for(i = 1; i <= n + 1; i++)
30     {
31         while(t && a[s[t]] > a[i]) r[s[t--]] = i;
32         s[++t] = i;
33     }
34     t = 0;
35     for(i = n; i >= 0; i--)
36     {
37         while(t && a[s[t]] > a[i]) l[s[t--]] = i;
38         s[++t] = i;
39     }
40     for(i = 1; i <= n; i++) ans = max(ans, a[i] * (r[i] - l[i] - 1));
41     printf("%lld\n", ans);
42     return 0;
43 }