96. 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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/**
 * 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 ArrayList<TreeNode> generateTrees(int n) {
    // Start typing your Java solution below
    // DO NOT write main() function
    return helper(1, n);
}

public ArrayList<TreeNode> helper(int s, int e){
    ArrayList<TreeNode> result = new ArrayList<TreeNode>();
    if(s == e){
        TreeNode r = new TreeNode(s);
        result.add(r);
        return result;
    }
    if(s > e){
        result.add(null);
        return result;
    }

    for(int i = s; i <= e; i++){
        ArrayList<TreeNode> left = helper(s, i-1);
        ArrayList<TreeNode> right = helper(i+1, e);

        for(TreeNode tl : left){
            for(TreeNode tr : right){
                TreeNode r = new TreeNode(i);
                r.left = tl;
                r.right = tr;
                result.add(r);
            }
        }
    }

    return result;
}
}