範例程式碼 uva10810

//uva10810
#include<bits/stdc++.h>

using namespace std;
int temp[500000];

long long int mergesort(int a[], int x, int y, long long int k) {
    if (y == x)
        return k;
    k = mergesort(a, x, (x+y)/2, k);
    k = mergesort(a, (x+y)/2+1, y, k);

    for (int i = x, i1 = (x+y)/2+1, j = (x+y)/2, j1 = y, l = 0;;) {
        if (i>j && i1>j1)
            break;
        else {
            if (i>j) {
                while (i1 <= j1) temp[l] = a[i1], i1++, l++;
            }
            else if (i1>j1) {
                while (i<=j) temp[l] = a[i], i++, l++;
            }
            else if (a[i] > a[i1]) {
                temp[l] = a[i1], k+= (j-i+1), l++, i1++;
            }
            else if (a[i] < a[i1]) {
                temp[l] = a[i], l++, i++;
            }
        }
    }
    for(int i = x, j = 0; i<=y; i++, j++)
        a[i] = temp[j];

    return k;
}
int main() {
    int n, a[500000];
    long long int k;

    while( scanf("%d",&n) && n) {
        k = 0;
        for(int i = 0; i<n; i++)
            scanf("%d", &a[i]);

        k = mergesort(a, 0, n-1, k);

        printf("%lld\n", k);
    }
    return 0;
}