37 lines
624 B
C++
37 lines
624 B
C++
#include<iostream>
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#include<set>
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#include<algorithm>
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using namespace std;
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int x[10009];
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int main()
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{
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int n,i,j;
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set<int>set1;
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while(scanf("%d",&n)!=EOF)
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{
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for(i=0;i<n;i++)
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{
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scanf("%d",&x[i]);
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set1.insert(x[i]);
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}
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sort(&x[0],&x[n]);
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int ok=0;
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for(i=0;i<n;i++)
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{
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for(j=i+2;j<n;j++)
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{
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if((x[i]+x[j])%2==0&&set1.find((x[i]+x[j])/2)!=set1.end())
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{
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ok=1;
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printf("Sequence is not 3-free. Witness: %d,%d,%d.\n",x[i],(x[i]+x[j])/2,x[j]);
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break;
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}
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}
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if(ok==1)break;
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}
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if(ok==0)
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printf("Sequence is 3-free.\n");
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}
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return 0;
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}
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