leetcode UniqueBST
z
 2019-04-09 10:36:27   
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import apple.laf.JRSUIUtils;
import utils.TreeNode;

import java.util.LinkedList;
import java.util.List;

public class UniqueBST {
    /*
    Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1 ... n.

    Example:

    Input: 3
    Output:
    [
      [1,null,3,2],
      [3,2,null,1],
      [3,1,null,null,2],
      [2,1,3],
      [1,null,2,null,3]
    ]
    Explanation:
    The above output corresponds to the 5 unique BST's shown below:

       1         3     3      2      1
        \       /     /      / \      \
         3     2     1      1   3      2
        /     /       \                 \
       2     1         2                 3
     */
    public static void main(String[] args){
        List<TreeNode> ans = uniqueBST(5);
        for (int i=0; i<ans.size(); i++){
            System.out.println(ans.get(i));
        }
    }
    public static List<TreeNode> generate_trees(int start, int end){
        List<TreeNode> ans = new LinkedList<>();
        if (start > end){
            return null;
        }
        if (start == end){
            ans.add(new TreeNode(start));
        }

        for (int i=start; i<=end; i++){
            TreeNode root = new TreeNode(i);
            List<TreeNode> left = generate_trees(start, i-1);
            List<TreeNode> right = generate_trees(i+1, end);
            for (TreeNode l: left){
                for(TreeNode r: right){
                    root.left = l;
                    root.right = r;
                }
            }
            ans.add(root);
        }
        return ans;
    }
    public static List<TreeNode> uniqueBST(int n){
        List<TreeNode> ans = new LinkedList<>();
        ans = generate_trees(1, n);
        return ans;
    }


}