HDU 1394 最小逆序數

#include <iostream>using namespace std;#define lson l, m , rt << 1#define rson m+1, r, rt << 1 | 1const int maxn = 5555;int sum[maxn<<2];void pushUp(int rt){sum[rt] = sum[rt<<1] + sum[rt<<1|1];}void build(int l, int r, int rt){sum[rt] = 0;if(l == r) return;int m = (l + r) >> 1;build(lson);build(rson);}void update(int p, int l, int r, int rt){if(l == r){sum[rt]++;return;}int m = (l +r) >> 1;if(p <= m) update(p, lson);else update(p, rson);pushUp(rt);}int query(int L, int R, int l, int r, int rt){if(L <= l && r <= R)return sum[rt];int m = (l + r) >> 1;int ret = 0;if(L <= m) ret += query(L, R, lson);if(R > m) ret += query(L, R, rson);return ret;}int x[maxn];int main(){int n;while(scanf("%d", &n) != EOF){build(0, n-1, 1);int sum = 0;for(int i = 0; i < n; i++){scanf("%d", &x[i]);sum += query(x[i], n-1, 0, n-1, 1);update(x[i], 0, n-1, 1);}int ret = sum;for(int i = 0; i < n; i++){sum += n - x[i] - x[i] - 1;ret = min(ret, sum);}printf("%d\n", ret);}return 0;}