14 lines Java DP solution


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    D
        public int numTrees(int n) {
            if (n==1 || n==0) return n;
            int[] dp = new int[n];
            dp[0] = 1;
            dp[1] = 2;
            for (int i = 2; i < n; i++) {
                dp[i] += dp[i-1]*2;
                for (int j = i-2; j >= Math.ceil((double) i / 2) - 1; j--) {
                    if (j == i - 2 - j) dp[i] += dp[j]*dp[j];
                    else dp[i] += dp[j]*dp[i - 2 - j]*2;
                }
            }
            return dp[n-1];
        }
    

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