class Solution {
public:
int numTrees(int n) {
vector<int>f(n+1,0);
f[0]=1,f[1]=1;
for(int i=2;i<=n;i++)
{
for(int k=1;k<=i;k++)
f[i]+=f[k-1]*f[i-k];
}
return f[n];
}
};
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