Leetcode 96. Unique Binary Search Trees

Question

Given n, how many structurally unique BST’s (binary search trees) that store values 1…n?

For example,

Given n = 3, there are a total of 5 unique BST’s.

1         3     3      2      1
\       /     /      / \      \
3     2     1      1   3      2
/     /       \                 \
2     1         2                 3

Code

public int numTrees(int n) {

if (n < 1) {
return 1;
}
int[] nums = new int[n + 1];
nums[0] = 1;
nums[1] = 1;
for (int i = 2; i <= n; i++) {
for (int k = 1; k <= i; k++) {
nums[i] += nums[k - 1] * nums[i - k];
}
}
return nums
;
}