poj 2299 树状数组 离散化

树状数组求逆序数+离散化

求逆序数时，加入当前数字，然后统计大于当前数字的数字个数，

题目数据很大，需先将其离散化。

#include<cstdio>#include<cstring>#include<algorithm>#define N 500005#define lowbit(x) (x&-x)using namespace std;int c[N], a[N];struct Discre{int pos, v;}disc[N];void add(int k, int sum) {while (k <= N) {c[k] += sum;k += lowbit(k);}}int sum(int k) {int res = 0;while (k) {res += c[k];k -= lowbit(k);}return res;}bool cam(Discre &a, Discre &b) {return a.v < b.v;}int main() {int n;while (EOF != scanf("%d", &n), n) {memset(c, 0, sizeof(c));for (int i = 1; i <= n; i++) {scanf("%d", &disc[i].v);disc[i].pos=i;}sort(disc + 1, disc + n + 1, cam);//离散化前，需将其排序for (int i = 1; i <= n; i++)a[disc[i].pos] = i;//离散化过程long long ans = 0;for (int i = 1; i <= n; i++) {add(a[i], 1);ans += sum(n) - sum(a[i]);}printf("%lld\n", ans);}return 0;}