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788. 逆序对的数量

https://www.acwing.com/problem/content/submission/code_detail/6595550/
This commit is contained in:
Baoshuo Ren 2021-07-22 07:57:21 +08:00 committed by Baoshuo Ren
parent d9969cbf64
commit 86a2d782d6
Signed by: baoshuo
GPG Key ID: 70F90A673FB1AB68

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AcWing/788/788.cpp Normal file
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#include <bits/stdc++.h>
using namespace std;
long long n, ans, a[500005], t[500005];
void merge_sort(long long a[], long long l, long long r) {
if (l >= r) return;
int mid = l + r >> 1;
merge_sort(a, l, mid);
merge_sort(a, mid + 1, r);
int i = l, j = mid + 1, k = 0;
while (i <= mid && j <= r) {
if (a[i] <= a[j]) {
t[k++] = a[i++];
} else {
t[k++] = a[j++];
ans += (mid - i + 1);
}
}
while (i <= mid) {
t[k++] = a[i++];
}
while (j <= r) {
t[k++] = a[j++];
}
for (int i = l, j = 0; i <= r; i++, j++) {
a[i] = t[j];
}
}
int main() {
cin >> n;
for (int i = 1; i <= n; i++) {
cin >> a[i];
}
merge_sort(a, 1, n);
cout << ans << endl;
return 0;
}