[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 = 2000002;
int n, t;
LL ans, a[MAXN], s[MAXN], l[MAXN], r[MAXN];

inline int read()
{
    int x = 0, f = 1;
    char ch = getchar();
    for(; !isdigit(ch); ch = getchar()) if(ch == '-') f = -1;
    for(; isdigit(ch); ch = getchar()) x = (x << 1) + (x << 3) + ch - '0';
    return x * f;
}

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

int main()
{
    int i;
    n = read();
    for(i = 1; i <= n; i++) a[i] = read();
    a[0] = a[n + 1] = -1;
    for(i = 1; i <= n + 1; i++)
    {
        while(t && a[s[t]] > a[i]) r[s[t--]] = i;
        s[++t] = i;
    }
    t = 0;
    for(i = n; i >= 0; i--)
    {
        while(t && a[s[t]] > a[i]) l[s[t--]] = i;
        s[++t] = i;
    }
    for(i = 1; i <= n; i++) ans = max(ans, a[i] * (r[i] - l[i] - 1));
    printf("%lld
", ans);
    return 0;
}