题目连接：点击打开链接
解题思路：
逆序数模板题。注意此题坑点在于数据大，，开成unsigned long long
完整代码：
#include <algorithm>#include <iostream>#include <cstring>#include <complex>#include <cstdio>#include <string>#include <cmath>#include <queue>using namespace std;typedef unsigned long long LL;const int MOD = int(1e9)+7;const int INF = 0x3f3f3f3f;const double EPS = 1e-9;const double PI = acos(-1.0); //M_PI;const int maxn = 2000001;int n;LL p[maxn] , t[maxn] , ans;void merge(int first , int last){int mid = (first + last) / 2;int i1 = 0 , i2 = first , i3 = mid + 1;while(i2 <= mid && i3 <= last){if(p[i2] > p[i3]){t[i1 ++] = p[i3 ++];ans += mid – i2 + 1;}elset[i1 ++] = p[i2 ++];}while(i2 <= mid) t[i1 ++] = p[i2 ++];while(i3 <= last) t[i1 ++] = p[i3 ++];i1 = first;i2 = 0;while(i2 < last – first + 1)p[i1 ++] = t[i2 ++];}void merge_sort(int first , int last){if(first < last){int mid = (last + first) / 2;merge_sort(first , mid);merge_sort(mid + 1 , last);merge(first , last);}}int main(){#ifdef DoubleQfreopen("in.txt","r",stdin);#endifwhile(cin >> n){ans = 0;memset(p , 0 , sizeof(p));memset(t , 0 , sizeof(t));for(int i = 0 ; i < n ; i ++){cin >> p[i];}merge_sort(0 , n – 1);cout << ans << endl;}return 0;}
爱上一个人的时候，总会有点害怕，怕得到他;怕失掉他。
