POJ 2299 Ultra-QuickSort（树状数组求逆序数）

http://poj.org/problem?id=2299

用树状数组求逆序数和用线段树求逆序数方法类似。
输入9 1 0 5 4，那么C[i]树状数组的建立是在，
下标 0 1 2 3 4 5 6 7 8 9
数组 1 1 0 0 1 1 0 0 0 1。
因此二者都需要离散化。
然后大概思路就是边更新边求值。

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#include <iostream>#include <cstdio>#include <cstring>#include <cmath>#include <set>#include <vector>#include <map>#include <queue>#include <stack>#include <algorithm>using namespace std;typedef long long LL;const int N = 500000 + 100;long long a[N],b[N];long long n;long long tr[N];long long lowbit(long long x){    return x & (-x);}void update(long long pos,long long val){    while(pos<=n)    {        tr[pos] += val;        pos += lowbit(pos);    }    return;}long long query(long long x){    long long sum = 0;    while(x)    {        sum += tr[x];        x -= lowbit(x);    }    return sum;}long long solve(){    long long res = 0;    for(long long i = n;i>=1;i--)    {        long long x = lower_bound(b+1,b+1+n,a[i])-(b+1) + 1;        res += query(x-1);        update(x,1);    }    return res;}int main(){    while(~scanf("%lld",&n) && n)     {        memset(tr,0,sizeof(tr));        memset(a,0,sizeof(a));        memset(b,0,sizeof(b));        for(long long i=1;i<=n;i++)            scanf("%lld",&a[i]),b[i] = a[i];        sort(b+1,b+1+n);        printf("%lld\n",solve());    }    return 0;}