【树状数组】cal 题解

求序列顺序对个数

#include <iostream>#include <sstream>#include <cstdio>#include <cstdlib>#include <ctime>#include <cmath>#include <cctype>#include <cstring>#include <algorithm>#ifndef WIN32#define Auto "%lld"#else#define Auto "%I64d"#endifusing namespace std;#define lowbit(__a) (__a & (-__a))typedef class IndexedTree {    public:        long long *lis;        int size;        IndexedTree() {     }        IndexedTree(int size):size(size) {            lis = new long long[(size + 1)];            memset(lis, 0, sizeof(long long) * (size + 1));        }        inline void add(int idx, long long x) {            for(; idx <= size; idx += lowbit(idx))                lis[idx] += x;        }        inline long long getSum(int idx) {            long long ret = 0;            for(; idx; idx -= lowbit(idx))                ret += lis[idx];            return ret;        }}IndexedTree;#define ll long longint n;int* arr;int* buf;int* f;inline void init() {    scanf("%d", &n);    arr = new int[(n + 1)];    buf = new int[(n + 1)];    f = new int[(n + 1)];    for(int i = 1; i <= n; i++)        scanf("%d", arr + i);}inline void discrete() {    memcpy(buf, arr, sizeof(int) * (n + 1));    sort(buf + 1, buf + n + 1);    for(int i = 1; i <= n; i++)        arr[i] = lower_bound(buf + 1, buf + n + 1, arr[i]) - buf;}IndexedTree it1, it2;ll res;inline void solve() {    it1 = IndexedTree(n);    it2 = IndexedTree(n);    for(int i = 1; i <= n; i++) {        it1.add(arr[i], 1);        f[i] = it1.getSum(arr[i] - 1);    }    for(int i = 1; i <= n; i++) {        it2.add(arr[i], f[i]);        res += it2.getSum(arr[i] - 1);    }    printf(Auto, res);}int main() {    init();    discrete();    solve();    return 0;}