题解
逆序对。。。1
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using namespace std;
typedef long long ll;
int read(){
int f = 1, x = 0;
char c = getchar();
while(c < '0' || c > '9'){if(c == '-') f = -f; c = getchar(); }
while(c >= '0' && c <= '9')x = x * 10 + c - '0', c = getchar();
return f * x;
}
int n, a[500005], b[500005];
ll ans;
void init(){
for(int i = 0; i < n; ++i)
a[i] = read();
}
void Merge(int ls, int rs, int re){
int lp = ls, rp = rs, tp = ls;
while(lp < rs && rp < re){
if(a[lp] <= a[rp]) b[tp++] = a[lp++];
else b[tp++] = a[rp++], ans += 1ll * (rs - lp);
}
while(lp < rs) b[tp++] = a[lp++];
while(rp < re) b[tp++] = a[rp++];
memcpy(a + ls, b + ls, sizeof(int) * (re - ls));
}
void ms(int l, int r){
int len = r - l, mid = (r + l) >> 1;
if(len == 1) return ;
ms(l, mid), ms(mid, r);
Merge(l, mid, r);
}
void solve(){
ans = 0;
ms(0, n);
printf("%lld\n", ans);
}
int main(){
while(n = read()){
init();
solve();
}
return 0;
}