POJ 2299

用归并排序求逆序对，用时391ms，但是用树状数组的话runtime error。

#include <stdio.h> 
#include <string.h> 
const int N=500000;
int a[N], b[N], n; 

__int64 Calc(int first, int last) 
{ 
    if (first == last-1) return 0; 
    int mid = (first+last)/2, i = first, j = mid, k = first;
      __int64  res = Calc(first, mid)+Calc(mid, last); 
    while (i < mid && j < last) 
    { 
        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 < last) b[k++] = a[j++]; 
    memcpy(a+first, b+first, sizeof(b[0])*(last-first)); 
    return res; 
} 

int main() 
{ 
   while( scanf("%d", &n),n){ 
    for (int i = 0; i < n; ++i) 
        scanf("%d", &a[i]); 
    printf("%I64d\n", Calc(0, n)); 
}
    return 0; 
} 

树状数组：

#include <stdio.h> 

const int N=500000;

int a[N+1]; 

void Inc(int x) 
{ 
    for (; x <= N; x += (x^x-1)&x) 
        ++a[x]; 
} 

__int64 Sum(int x) 
{ 
    __int64 res = 0; 
    for (; x; x -= (x^x-1)&x) 
        res += a[x]; 
    return res; 
} 

int main() 
{ 
    int n;__int64 res; 
while(scanf("%d",&n)!=EOF,n)
{
res=0;
    while(n--)
    { 
        int t; 
        scanf("%d", &t); 
        res += Sum(N-t); 
        Inc(N-t+1); 
    } 
    printf("%I64d\n", res); 
}
    return 0; 
}