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37 lines
816 B
37 lines
816 B
#include <bits/stdc++.h>
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using namespace std;
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typedef long long LL;
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const int N = 100010;
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int n; // n个数据
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int q[N]; // 原数组
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int t[N]; // 临时数组
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LL ans; // 结果
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void merge_sort(int q[], int l, int r) {
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if (l >= r) return;
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int mid = (l + r) >> 1;
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merge_sort(q, l, mid);
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merge_sort(q, 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 (q[i] <= q[j])
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t[k++] = q[i++];
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else {
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t[k++] = q[j++];
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ans += mid - i + 1;
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}
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while (i <= mid) t[k++] = q[i++];
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while (j <= r) t[k++] = q[j++];
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for (i = l, j = 0; i <= r; i++, j++) q[i] = t[j];
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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++) cin >> q[i];
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merge_sort(q, 1, n);
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printf("%lld\n", ans);
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return 0;
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} |