BZOJ 1597: [Usaco2008 Mar]土地购买 动态规划 + 斜率优化

Code: 

#include<bits/stdc++.h> 
#define maxn 1000000 
#define ll long long 
#define x(i) (b[i+1]) 
#define y(i) (f[i]) 
#define setIO(s) freopen(s".in","r",stdin) 
using namespace std;  
int A[maxn],arr[maxn],q[maxn],tot,head,tail;  
ll a[maxn], b[maxn], f[maxn], u[maxn], v[maxn];         
bool cmp(int i,int j) 
{
    return u[i]==u[j]?v[i]<v[j]:u[i]<u[j]; 
}
double slope(int i,int j)
{
    return (double)(1.00*y(i)-y(j))/(double)(1.00*x(j)-x(i));       
}
int main()
{
    // setIO("input"); 
    int n,i,j; 
    scanf("%d",&n);   
    for(i=1;i<=n;++i) scanf("%lld%lld",&u[i],&v[i]),A[i]=i; 
    sort(A+1,A+1+n,cmp);   
    for(i=1;i<=n;++i) 
    { 
        while(v[A[i]]>=b[tot]&&tot) --tot; 
        ++tot, a[tot]=u[A[i]], b[tot]=v[A[i]];  
    }
    n=tot,head=tail=0;  
    for(i=1;i<=n;++i) 
    {           
        while(head<tail&&slope(q[head],q[head+1])<a[i]) ++head; 
        f[i]=f[q[head]]+a[i]*b[q[head]+1];               
        while(head<tail&&slope(q[tail], i)<slope(q[tail-1],i)) --tail; 
        q[++tail]=i; 
    }
    printf("%lld\n",f[n]);     
    return 0; 
}