POJ-2299 Ultra-QuickSort 逆序對統計

基本的樹狀數組求逆序對

// Ultra-QuickSort
// http://poj.org/problem?id=2299

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#define forloop(i, a, b) for(int i = a; i < b; i++)
#define fordown(i, a, b) for(int i = a; i > b; i--)

using namespace std;

const int maxn = 500010;
typedef long long LL;

struct Node {
    int val, id;
};

int n;
Node a[maxn];
int arr[maxn];

inline int lowbit(int x) {
    return x & -x;
}

void add(int x, int v) {
    for (int i = x; i < n + 4; i += lowbit(i)) {
        arr[i] += v;
    }
}

LL sum(int x) {
    LL s = 0;
    for (int i = x; i ; i -= lowbit(i)) {
        s += arr[i];
    }
    return s;
}

struct NodeCmp {
    bool operator() (const Node& l, const Node& r) {
        return l.val < r.val;
    }
};

int main() {
    while (cin >> n) {
        if (n == 0) break;
        memset(arr, 0, sizeof(int) * (n + 10));
        forloop(i, 1, n+1) {
            scanf("%d", &a[i].val);
            a[i].id = i;
        }
        sort(a+1, a+1+n, NodeCmp());
        LL ans = 0;
        fordown(i, n, 0) {
            ans += sum(a[i].id - 1);
            add(a[i].id, 1);
        }
        cout << ans << endl;
    }
    return 0;
}