[Leetcode] Unique Binary Search Trees II (Java)

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

列出所有这种数，树的题好多的DFS

public class Solution {   private ArrayList<TreeNode> generate(int begin,int end){ArrayList<TreeNode> res = new ArrayList<TreeNode>();if(begin>end){res.add(null);return res;}for(int i=begin;i<=end;i++){ArrayList<TreeNode> left =generate(begin, i-1);ArrayList<TreeNode> right = generate(i+1, end);for(int l=0;l<left.size();l++){for(int r=0;r<right.size();r++){TreeNode root = new TreeNode(i+1);root.left = left.get(l);root.right = right.get(r);res.add(root);}}}return res;}public ArrayList<TreeNode> generateTrees(int n) {return generate(0, n-1);}}