POJ2299 UltraQuickSort

　　原题传送：http://poj.org/problem?id=2299

　　归并排序求逆序数，nlog(n)，500多ms。答案最大可以达到n * (n + 1) / 2，要开long long。

 #include <stdio.h>
 #include <stdlib.h>
 #define N 500005
 #define LL __int64
 LL a[N], b[N];
 int n;
 
 LL cal(LL s, LL t)
 {
     LL i, j, k, mid;
     LL res;
     if(t - s == 1)
         return 0;
     mid = (s + t) >> 1;
     res = cal(s, mid) + cal(mid, t);
     i = s, j = mid, k = s;
     while(i < mid && j < t)
     {
         if(a[i] <= a[j])
             b[k ++] = a[i ++], res += j - mid;
         else
             b[k ++] = a[j ++];
     }
     while(i < mid) b[k ++] = a[i ++], res += j - mid;
     while(j < t) b[k ++] = a[j ++];
     for(i = 0; i < t - s; i ++)
         a[s + i]= b[s + i];
     return res;
 }
 
 int main()
 {
     while(scanf("%d", &n), n)
     {
         for(LL i = 0; i < n; i ++)
             scanf("%I64d", &a[i]);
         printf("%I64d\n", cal(0, n));
     }
     return 0;
 }

　　POJ1007用的是一样的算法。