POJ 2299 - Ultra-QuickSort

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传送门POJ 2299 - Ultra-QuickSort

小白上的求逆序数的例题。。还是略有不懂。晚上再想想。。

#include <cstdio>#include <cstring>#include <algorithm>using namespace std;long long cnt, a[510000], t[510000];void MergeSort(long long *a, int x, int y, long long *t){    if (y - x > 1)    {        int m = x + (y - x) / 2;        int p = x, q = m, i = x;        MergeSort(a, x, m, t);        MergeSort(a, m, y, t);        while (p < m || q < y)        {            if (q >= y || (p < m && a[p] <= a[q]))                t[i++] = a[p++];            else            {                t[i++] = a[q++];                cnt += m - p;            }        }        for (i = x; i < y; i++)            a[i] = t[i];    }}int main(){    //freopen("input.txt", "r", stdin);    int n, i, j;    while (scanf("%d", &n), n)    {        for (i = 0; i < n; i++)            scanf("%lld", &a[i]);        cnt = 0;        MergeSort(a, 0, n, t);        printf("%lld\n", cnt);    }    return 0;}

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