Leetcode-Unique Binary Search Trees II

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

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

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Solution:

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; left = null; right = null; }
 * }
 */
public class Solution {
    public List<TreeNode> generateTrees(int n) {
        List<TreeNode> list = generateTreesRecur(n,1,n);
        return list;
        
    }

    public List<TreeNode> generateTreesRecur(int len, int s, int e){
        List<TreeNode> treeList = new ArrayList<TreeNode>();
        if (len==0){
            treeList.add(null);
            return treeList;
        }

        if (len==1){
            TreeNode newNode = new TreeNode(s);
            treeList.add(newNode);
            return treeList;
        }

        for (int i=s;i<=e;i++){
            int leftLen = i-s;
            int rightLen = e-i;
            List<TreeNode> leftList = generateTreesRecur(leftLen,s,i-1);
            List<TreeNode> rightList = generateTreesRecur(rightLen,i+1,e);
            for (int j=0;j<leftList.size();j++)
                for (int k=0;k<rightList.size();k++){
                    TreeNode root = new TreeNode(i);
                    root.left = leftList.get(j);
                    root.right = rightList.get(k);
                    treeList.add(root);
                }

        }

        return treeList;

   }
}