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