poj3040 Allowance

思路：

贪心。

看了题解说是

先把面值从大到小排序
然后从头往尾扫，只要不超额，能取多少去多少
然后如果还有剩余，就从尾往头扫，尽量取，让他恰好超额

不过并不懂证明。

实现：

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

int N, C;
struct node
{
    int d, c;
};
node a[25];

bool cmp(const node & a, const node & b)
{    
    return a.d > b.d;
}

int main()
{
    cin >> N >> C;
    for (int i = 0; i < N; i++)
    {
        scanf("%d %d", &a[i].d, &a[i].c);
    }
    sort(a, a + N, cmp);
    int cnt = 0;
    int j = 0;
    for (; j < N; j++)
    {
        if (a[j].d < C)
            break;
    }
    for (int i = 0; i < j; i++)
    {
        cnt += (a[i].d / C) * a[i].c;
    }
    while (true)
    {
        int now = 0;
        for (int i = j; i < N; i++)
        {
            while (a[i].c && now + a[i].d <= C)
            {
                now += a[i].d;
                a[i].c--;
            }
        }
        for (int i = N - 1; i >= j; i--)
        {
            while (a[i].c && now < C)
            {
                now += a[i].d;
                a[i].c--;
            }
        }
        if (now < C)
            break;
        cnt++;
    }
    cout << cnt << endl;
    return 0;
}