31 lines
559 B
C++
31 lines
559 B
C++
#include<cstdio>
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#include<cstring>
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const int maxn=5e5+5;
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int n; long long res;
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int a[maxn],b[maxn];
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void GB(int *a,int *b,int len)
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{
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if(len<=1) return;
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int M=len/2,p1=0,p2=M,p=0;
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GB(a,b,M); GB(a+M,b+M,len-M);
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while(p1<M&&p2<len)
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{
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if(a[p1]<=a[p2]) b[p++]=a[p1++];
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else res+=M-p1,b[p++]=a[p2++];
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}
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while(p1<M) b[p++]=a[p1++];
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while(p2<len) b[p++]=a[p2++];
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memcpy(a,b,len<<2);
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}
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int main()
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{
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#ifdef local
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freopen("pro.in","r",stdin);
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#endif
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scanf("%d",&n);
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for(int i=0;i<n;i++) scanf("%d",&a[i]);
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GB(a,b,n);
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printf("%lld\n",res);
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return 0;
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}
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