public class Solution {
public int numTrees(int n) {
if(n<2) return n;
int dp[] = new int[n+1];
dp[0] = 1;
dp[1] = 1;
for(int i=2; i<=n; i++) {
for(int j=0; j<i/2; j++) {
dp[i] += 2*dp[j]*dp[i-1-j];
}
if(i % 2 != 0) dp[i] += dp[i/2] * dp[i/2];
}
return dp[n];
}
}
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