Unique Binary Search Trees
给一个数字, 问能组成多少个唯一的BST. 卡特兰数
public class Solution {
public int numTrees(int n) {
int[] dp = new int[n+1];
dp[0] = 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];
}
}