Poison

96. Unique Binary Search Trees

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class Solution {
    public int numTrees(int n) {
        if (n <= 2) {
            return n;
        }
        
        int[] dp = new int[n + 1];
        dp[0] = 1;
        dp[1] = 1;
        dp[2] = 2;

        for (int i = 3; i <= n; i++) {
            for (int root = 1; root <= i; root++) {
                dp[i] += dp[root - 1] * dp[i - root]; // cartesian product
            }
        }

        return dp[n];
    }
}
Reference

96. Unique Binary Search Trees