Question

link

Given n, generate all structurally unique BST's (binary search trees) that store values 1...n.

For example,
Given n = 3, your program should return all 5 unique BST's shown below. 

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

Stats

Frequency 1
Difficulty 4
Adjusted Difficulty 3
Time to use --------

Ratings/Color = 1(white) 2(lime) 3(yellow) 4/5(red)

Analysis

This is a question with standard solution.

It looks like difficult, but actually the code is very short.

Solution

This is my previous solution. Everyone in blogs are having same solution. I mean, every single one of them, one example.

Code

public ArrayList<TreeNode> generateTrees(int n) {
    return construct(0, n);
}

ArrayList<TreeNode> construct(int base, int n) {
    ArrayList<TreeNode> ans = new ArrayList<TreeNode>();
    if (n == 0) ans.add(null);
    for (int cur = 1; cur <= n; cur++)
        for (TreeNode l : construct(base, cur - 1))
            for (TreeNode r : construct(base + cur, n - cur)) {
                TreeNode root = new TreeNode(base + cur);
                root.left = l;
                root.right = r;
                ans.add(root);
            }
    return ans;
}