public static int numTrees(int n) {
int[] dp = new int[n+1];
dp[0] = 1;
dp[1] = 1;
for (int i = 2; i <= n; i++) {
for (int j = 1; j <= i; j++) {
dp[i]+=dp[j-1]*dp[i-j];
}
}
return dp[n];
}
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