Unique Binary Search Trees - Catalan Numbers
Given an integer n, return the number of structurally unique BST's (binary search trees) which has exactly n nodes of unique values from 1 to n.
public int numTrees(int n) {
int [] dp = new int[n+1];
dp[0]= 1;
dp[1] = 1;
for(int level = 2; level <=n; level++)
for(int root = 1; root<=level; root++)
dp[level] += dp[level-root]*dp[root-1];
return dp[n];
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