poj3253 Fence Repair

Time Limit: 2000MS		Memory Limit: 65536K
Total Submissions: 95562		Accepted: 31122

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 Li (1 ≤ Li ≤ 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 Li). 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).

#include<iostream>
#include<queue>
#define ll long long
using namespace std;

struct cmp {
	bool operator()(int a, int b) {
		return a > b;    //Minimum value priority
	}
};

int main() {
	priority_queue<int, vector<int>, cmp > q;
	ll sum = 0;
	int n;
	cin >> n;
	//Read in data
	while (n--) {
		int a;
		cin >> a;
		q.push(a);
	}
	if (q.size() == 1) {
		cout << q.top();
		return 0;
	}
	while (q.size() > 1) {//Stop when there is one node left in the queue
		ll i = q.top(); q.pop();
		ll j = q.top(); q.pop();
		sum += i + j;
		q.push(i + j);
	}
	cout << sum<<endl;
	return 0;
}

Conclusion：

According to this problem we can easily observe that the problem is to build Huffman tree, through the priority queue we can quickly find the exit of the problem.

priority_queue

Headers：

        #include<queue>

Definition:

        priority_queue<int>pq;

Operation:

        pq.empty() //Determine if the queue is empty

        pq.pop() //Delete the top element of the queue

        pq.push() //Add an element

        pq.size() //Returns the number of elements in the queue

        pq.top() //Return the top element of the queue

Modify sort mode

The priority queue is sorted from largest to smallest by default, i.e. the larger the element value, the higher the priority. 

# use a comparison structure with a custom priority
struct cmp{
    bool operator ()(int a, int b){
        return a>b;    //Minimum value priority
    }
};

priority_queue<int, vector<int>, cmp > q;