为什么用树状数组求逆序对会全部RE

rt，求大佬解答

#include<cstdio>
#include<cstring>
#define re register int
using namespace std;
typedef long long ll;
const int maxn=510000;
ll ans;
int n,A[maxn],c[maxn];
inline int read() {
    int x=0,cf=1;
    char ch=getchar();
    while(ch<'0'||ch>'9') {
        if(ch=='-') cf=-1;
        ch=getchar();
    }
    while(ch>='0'&&ch<='9') {
        x=(x<<3)+(x<<1)+(ch^48);
        ch=getchar();
    }
    return x*cf;
}
int query(int x) {
    int res=0;
    for(;x;x-=x&-x) res+=c[x];
    return res;
}
void add(int x,int v) {
    for(;x<=500000;x+=x&-x) c[x]+=v;
}
int main() {
    n=read();
    for(re i=1;i<=n;i++) A[i]=read();
    for(re i=1;i<=n;i++) {
        add(A[i],1);
        ans+=i-query(A[i]);
    }
    printf("%lld",ans);
    return 0;
}