ALDS1_5_D 2018-2-24

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#include<iostream>
#include<cstdio>
using namespace std;
const int MAX = 500000;
const int maxnum = 1000000010;// 两个局部数组
int L[MAX / 2 + 2], R[MAX / 2 + 2];
int A[MAX], n;
long long cnt ;// 排序和合并
void merge(int left, int mid, int right) {int n1 = mid - left;int n2 = right - mid;for (int i = 0; i < n1; i++) L[i] = A[left + i];for (int i = 0; i < n2; i++) R[i] = A[mid + i];// 两个局部数组最后添加一个比较大的数L[n1] = R[n2] = maxnum;int i = 0, j = 0;// 合并for (int k = left; k < right; k++) {if (L[i] <= R[j])A[k] = L[i++];else {A[k] = R[j++];cnt += n1 - i;}}
}
// 分割
void mergeSort(int left, int right) {if (left + 1 < right) {int mid = (left + right) / 2;mergeSort(left, mid);mergeSort(mid, right);merge(left, mid, right);}
}
int main() {scanf("%d", &n);for (int i = 0; i < n; i++)scanf("%d", &A[i]);mergeSort(0, n);printf("%lld\n", cnt);return 0;
}

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