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.
This method to this problem is similar to Unique Binary Search Trees.
public class Solution { public ArrayList<TreeNode> generateTrees(int n) { return treeHelp(1, n); } public ArrayList<TreeNode> treeHelp(int l, int n){ ArrayList<TreeNode> result = new ArrayList<TreeNode>(); if(l > n){ // no TreeNode can be generated result.add(null); return result; } else{ for(int i = l; i <= n; ++i){ ArrayList<TreeNode> leftNode = treeHelp(l, i - 1); ArrayList<TreeNode> rightNode = treeHelp(i + 1, n); for(int j = 0; j < leftNode.size(); ++j){ for(int m = 0; m < rightNode.size(); ++m){ TreeNode root = new TreeNode(i); root.left = leftNode.get(j); root.right = rightNode.get(m); result.add(root); } } } } return result; } }