SGU 180

题型：求逆序数，（用树状数组+离散化）

心得：很悲剧，TLE AT #31 N 次，原因出在qsort()，（难怪发现不了），后来改sort()，就过了。看来qsort()的效率还有待分析！

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define NL 65538
#define LL __int64

struct Num {
    int v, no;
}a[NL];
int n, b[NL], mx;
int t[NL];

int cmp(const void *a, const void *b)
{
    return ((struct Num *)a)->v - ((struct Num *)b)->v;
}

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

int sum(int k)
{
    int cnt = 0;
    while (k > 0) {
        cnt += t[k];
        k -= lowbit(k);
    }
    return cnt;
}

void update(int k, int c)
{
    while (k <= mx) {
        t[k] += c;
        k += lowbit(k);
    }
}

int main()
{
    int i, p;
    LL cnt;
    //    freopen("in.txt", "r", stdin);
    //    freopen("out.txt", "w", stdout);
    scanf("%d", &n);
    for (i=0; i<n; i++) {
        scanf("%d", &a[i].v);
        a[i].no = i;
    }
    qsort(a, n, sizeof(a[0]), cmp);
    p=1;
    b[a[0].no] = 1;
    for (i=1; i<n; i++) {
        if (a[i].v != a[i-1].v) p++;
        b[a[i].no] = p;
    }
    mx = p;
    memset(t, 0, sizeof(t));
    cnt = 0;
    for (i=n-1; i>=0; i--) {
        cnt += sum(b[i]-1);
        update(b[i], 1);
    }
    printf("%I64d\n", cnt);
    return 0;
    
}