Solution.java

class Solution {
    public int numTrees(int n) {

        // res[n] 表示整数n组成的二叉搜索树个数
        int[] res = new int[n + 1];
        res[0] = 1;

        for (int i = 1; i <= n; ++i) {
            for (int j = 0; j < i; ++j) {
                res[i] += res[j] * res[i - j - 1];
            }
        }
        return res[n];
    }
}