```
public class Solution {
public int numTrees(int n) {
int[] dp = new int[n + 2];
dp[0] = dp[1] = 1;
for (int len = 2; len <= n; len++) { // length
for (int j = 1; j <= len; j++) { // j as root
// j - 1: left child length
// i - j: right child length
dp[len] += dp[j - 1] * dp[len - j];
}
}
return dp[n];
}
}
```