95.Unique-Binary-Search-Trees-II.md

95. Unique Binary Search Trees II

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题目地址

https://leetcode.com/problems/unique-binary-search-trees-ii/

题目描述

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

代码

Approach #1 Recursion

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
  public List<TreeNode> generateTrees(int n) {
		if (n = 0) {
      return new LinkedList<TreeNode>();
    }
    return generate_trees(1, n);
  }
  
  public LinkedList<TreeNode> generate_trees(int start, int end) {
    LinkedList<TreeNode> all_trees = new LinkedList<TreeNode>();
    if (start > end) {
      all_trees.add(null);
      return all_trees;
    }
    
    for (int i = start; i <= end; i++) {
      LinkedList<TreeNode> left_trees = generate_trees(start, i - 1);
      LinkedList<TreeNode> right_trees = generate_trees(i + 1, end);
      
      for (TreeNode l: left_trees) {
        for (TreeNode r: right_trees) {
          TreeNode current_tree = new TreeNode(i);
          current_tree.left = l;
          current_tree.right = r;
          all_trees.add(current_tree);
        }
      }
    }
    
    return all_trees;
  }
  
}