求逆序數 ?----》 離散化

#include<stdio.h> #include<string.h> #include<algorithm> #include<iostream> using namespace std; #define M 1000005 typedef __int64 LL; LL a[M]; int n; struct node {LL x,y; }s[M]; bool cmp(node p,node q) {return p.x < q.x; } LL lowbit(LL x) {return x&(-x); } void add(LL pos,LL num) {while(pos <= n){a[pos]+=num;pos += lowbit(pos);} } LL getsum(LL pos) {LL res = 0;while(pos > 0){res += a[pos];pos -= lowbit(pos);}return res; } int main() {while(scanf("%d",&n)!=EOF){memset(a,0,sizeof(a));for(LL i = 1;i <= n;i++){scanf("%I64d",&s[i].x);s[i].y = i;}sort(s+1,s+n+1,cmp);LL ret = 0;for(LL i = 1;i <= n;i++){add(s[i].y,1);ret += i - getsum(s[i].y);}printf("%I64d\n",ret);} }

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