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[];

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

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