poj2513 Fence Repair(小根堆)

Description

Farmer John wants to repair a small length of the fence around the pasture. He measures the fence and finds that he needs N (1 ≤ N ≤ 20,000) planks of wood, each having some integer length L_i (1 ≤ L_i ≤ 50,000) units. He then purchases a single long board just long enough to saw into the N planks (i.e., whose length is the sum of the lengths L_i). FJ is ignoring the "kerf", the extra length lost to sawdust when a sawcut is made; you should ignore it, too.

FJ sadly realizes that he doesn't own a saw with which to cut the wood, so he mosies over to Farmer Don's Farm with this long board and politely asks if he may borrow a saw.

Farmer Don, a closet capitalist, doesn't lend FJ a saw but instead offers to charge Farmer John for each of the N-1 cuts in the plank. The charge to cut a piece of wood is exactly equal to its length. Cutting a plank of length 21 costs 21 cents.

Farmer Don then lets Farmer John decide the order and locations to cut the plank. Help Farmer John determine the minimum amount of money he can spend to create the N planks. FJ knows that he can cut the board in various different orders which will result in different charges since the resulting intermediate planks are of different lengths.

Input

Line 1: One integer N, the number of planks 
Lines 2..N+1: Each line contains a single integer describing the length of a needed plank

Output

Line 1: One integer: the minimum amount of money he must spend to make N-1 cuts

Sample Input

3
8
5
8

Sample Output

34

Hint

He wants to cut a board of length 21 into pieces of lengths 8, 5, and 8. 
The original board measures 8+5+8=21. The first cut will cost 21, and should be used to cut the board into pieces measuring 13 and 8. The second cut will cost 13, and should be used to cut the 13 into 8 and 5. This would cost 21+13=34. If the 21 was cut into 16 and 5 instead, the second cut would cost 16 for a total of 37 (which is more than 34).

题意：给出x个数，合并两个数的代价为两数之和，求合并所有数字的最小代价

题解：英文版的合并果子……将所有数先装进小根堆，然后每次取出两个最小的，加完后再放回堆中即可，对了，记得用long long！

 

代码如下：

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

int n;

struct Heap
{
    long long a[30000];
    int sz1;
    
    void clear()
    {
        sz1=1;
    }
    
    void push(long long x)
    {
        int i=sz1++;
        while(i>1)
        {
            int p=(i>>1);
            if(a[p]<x)
            {
                break;
            }
            a[i]=a[p];
            i=p;
        }
        a[i]=x;
    }
    
    long long top()
    {
        return a[1];
    }
    
    long long pop()
    {
        int ans=a[1];
        int x=a[--sz1];
        int i=1;
        while((i<<1)<sz1)
        {
            int ls=i<<1;
            int rs=i<<1|1;
            if(rs<sz1&&a[rs]<a[ls])
            {
                ls=rs;
            }
            if(a[ls]>x)
            {
                break;
            }
            a[i]=a[ls];
            i=ls;
        }
        a[i]=x;
        return ans;
    }
    
    int sz()
    {
        return sz1;
    }
}heap;

int main()
{
    long long b,ans=0;
    scanf("%d",&n);
    heap.clear();
    for(int i=1;i<=n;i++)
    {
        scanf("%lld",&b);
        heap.push(b);
    }
    while(heap.sz()>2)
    {
        long long x=heap.pop();
        long long y=heap.pop();
        ans+=x+y;
        heap.push(x+y);
    }
    printf("%lld\n",ans);
}