归并排序

归并排序：

#include <bits/stdc++.h>
using namespace std;

const int maxn = 1e5 + 100;

long long ans;
int data[maxn], t[maxn];

void MergeSort(int l, int r) {
    if(l >= r) return ;
    int mid = (l + r) / 2;
    MergeSort(l, mid);
    MergeSort(mid + 1, r);

    int i = l, j = mid + 1, cnt = 0;
    while(i <= mid && j <= r) {
        if(data[i] <= data[j]) {
            t[cnt++] = data[i++];
        } else {
           t[cnt++] = data[j++];
           ans += mid - i + 1;
        }
    }

    while(i <= mid) t[cnt++] = data[i++];
    while(j <= r) t[cnt++] = data[j++];

    for(int i = 0; i < cnt; i++)
        data[l + i] = t[i];
}

int main()
{
    int n, m;
    cin >> n;
    for(int i = 0; i < n; i++) {
        scanf("%d", &data[i]);
    }

    MergeSort(0, n - 1);

    printf("%lld\n", ans);
}