poj3250

利用栈进行动态规划，要注意的地方是总和得用longlong因为80000^2是超出int范围的。

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#include <cstdio>
#include <iostream>
#include <cstdlib>
#include <cstring>
using namespace std;

#define maxn 80002

struct item
{
int num, h;
}stk[maxn];

int cow[maxn], f[maxn], front = 0;

int main()
{
//freopen("D:\\t.txt", "r", stdin);
    int n;
    scanf("%d", &n);
for (int i = 0; i < n; i++)
        scanf("%d", &cow[i]);
    stk[front].num = n - 1;
    stk[front++].h = cow[n - 1];
for (int i = n - 2; i >= 0; i--)
    {
while (cow[i] > stk[front - 1]. h && front > 0)
            front--;
if (front == 0)
        {
            f[i] = n - i - 1;
            stk[front].num = i;
            stk[front++].h = cow[i];
continue;
        }
        f[i] = stk[front - 1]. num - i - 1;
        stk[front].num = i;
        stk[front++].h = cow[i];
    }
long long sum = 0;
for (int i = 0; i < n; i++)
        sum += f[i];
    printf("%I64d\n", sum);
return 0;
}