zoj2883

题目大意：

商品买3免1，问怎么买可以最大优惠。

解题思路：

简单排个序，三个一组，把每组最后一件商品价格相加就好了。

代码如下：

#include <stdio.h>
int P[20010];
void merge(int * a, int start, int mid, int end);
void merge_sort(int * a, int start, int end);
int main(void)
{
 int t, n, i, j, tot, temp;

 scanf("%d", &t);
 while (t--)
 {
  scanf("%d", &n);
  for (i = 0; i < n; i++)
   scanf("%d", &P[i]);

        merge_sort(P, 0, n-1);

  tot = 0;
  for (i = 2; i < n; i += 3)
     tot += P[i];

  printf("%d", tot);
  if (t) printf("\n");
 }
 return 0;
}

void merge_sort(int * a, int start, int end)
{
 int mid;

 if (start < end)
    {
  mid = (start + end) / 2;
  merge_sort(a, start, mid);
  merge_sort(a, mid+1, end);
  merge(a, start, mid, end);
 }
}

void merge(int * a, int start, int mid, int end)
{
 int i, j, k;
 int n1 = mid - start + 1;
 int n2 = end - mid;

 int L[n1+1], R[n2+1];

 for (i = 0; i < n1; i++)
 {
  L[i] = a[start + i];
 }

 for (j = 0; j < n2; j++)
 {
     R[j] = a[mid + 1 + j];
 }

 L[n1] = -1;
 R[n2] = -1;  

 i = j = 0;
 for (k = start; k <= end; k++)
 {
  if (L[i] > R[j])  a[k] = L[i++];
  else a[k] = R[j++];
 }
}